RGB LED strips are a great way to inconspicuously add color in places, they typically come with adhesive backing to make it easier to stick them anywhere. These LED strips have become fairly easy and inexpensive to buy, and folks are still finding lots of interesting places for them. There are many kinds of LED strips, but the more common ones are the RGB 5 meter strips that have a IR remote, and the controller runs on 12v, and the LEDs are typically common anode. The strip requires 4 wires, +12v, and 3 grounds for the Red, Green, and Blue LEDs. Normally in cars, this is an ideal set up, because cars have big 12v batteries, and typically the + wire is switched (to turn lights on, or power a device).
My car has the door lights switched to ground, which is the opposite of what I need. To make the door lights come on when the door is opened, I have to switch the +12v on, and the LED controller does the sinking to ground. Since the switch switches to ground, I would need 3 switches (one for red, one for green, and one for blue). Thats just not practical. There is another way to solve this problem, and that is to invert the polarity, In cars its more common to use a relay to do that sort of thing, but in electronics we like to use MOSFETs to do that sort of thing, and I have some that didnt work for another use, but are perfect for this.
I took my dash cover off, and hooked my meter up to the switches, and found where the driving lights sourced 12v when turned on. I tapped in there, since it was close to my dash. I mounted my controller under the dash, It can be easily changed, or use the remote to change the lights. The controller always gets 12v, but the dash only gets 12v when the lights are on, and the doors are only on when each door is open. I wanted to hook up the dome light too, but the wiring was more trouble than i wanted to deal with.

There are several components to this document, this part is the overview, the next part will be building controller, mounting lights in the dash, making the wiring cables, building the door lights, building the inversion circuit for the door light switches, installing the under door LEDs, and maybe a little bit of an overview.

Step 1: Building the Controller

The controller is an inexpensive RGB LED controller, I actually broke a few of these when setting this up, its good that they are fairly cheap now. The first one I used is is the kind that have a little white box, and I soldered wires directly to it for the power, and for the output. I ended up changing it over to a smaller style that has no box, and I just made a connector for it instead of soldering wires directly to it.
The pictures should he basic idea of how the electronics work, you can hook it up any way that you like, use fancy connectors, or be creative and make your own, or just solder stuff together, whatever works. There are many controllers and LED strips to chose from, any of them will be an improvement, so do it how and with what you like. If you are using IR remote unit like I do, then you have to keep the receiver in view, dont bury it under your dash, or glove box. You could also make a controller with some mosfets, resistors, and an arduino, there are lots of examples of ways to build your own LED strip controller. I could have done that, but I prefer the off the shelf thing with remote.
The controller should be wired so that it always has power, and is always running, the lights are switched on only when you want them, but the controller is always running. Its so basic and simple that I wont provide a schematic.

Step 2: Mounting the LEDs in the Dash

This part is a little scary for most folks, taking that dash out is a little daunting, nobody wants to break their car. My car is 30 years old, and it has the added problem of dried up brittle plastic, which may break if you look at it funny.

I had to redo mine a couple times. I thought one little strip on the inside of the clear dash cover was going to work, but it lights the whole display, and makes it harder to see the gauges. What you need is LEDs inside the gauges, with the flat black cover to light only the gauges, not the entire display.

I ended up breaking the plastic surround that goes on top of the display, I had a heck of a time trying to find a replacement, and managed to patch it back together with bondo and plumbers tape. I recommend being very careful with your dash parts, especially if they are made of old plastic.

I took the hood off, and that exposes the buttons, and I was able to tap into the switch for the running lights, when they are on, the dash lights should be on. I made a small 3 wire cable, and ran it to the controller, and for the positive, I ran it to the switch. and I made a little connector to hook it on to the LEDs. Its a simple enough circuit, exactly as you might expect it to be, 4 wires, 3 go to the controllers R,G,B outputs and 1 to the switch.
I removed the dash, and took it apart to expose the inner gauge area, and put non waterproof LEDS (they dont need waterproofing in the dash, and the non waterproof are more flexible, and easy to install.)
To fit the LEDs in the dash, you lay them out how you want them, cut the length, and put the basic bends and form it to fit the area that you want. Then pull it out, take the backing off the sticky part, and gently place it where it where you want it. This are is sealed, and everything was very clean, so I didnt prep the surface, I just stuck them in place. I think I added some super glue in the corners to help it stay in place even if the glue got warm and soft.
The important thing here is that the LEDs stay in place, and that you can get your 4 wires outside after you seal them in. I put a connector on mine, so that I could unplug it, and remove the dash.

I left all the bulbs in, my car has a dimmer, so I can just dim the stock bulbs if I want, most have burned out anyway. I could have fitted each bulb with RGB LED, and wired them up, but sticking a strip all around the gauges worked just as good, and was much easier then fussing with each bulb. I probably should have replaced the indicator bulbs with white LEDs, because bulbs fail, LEDs last a very long time.

There are a couple things you should keep in mind when messing with your dash. The area inside the clear plastic is usually air tight, clean, and dust free. When working on your dash, try to prevent dust from getting in, and dont touch the inside area, or you will leave fingerprints, or need to clean,and if you try to clean it, you will leave dust, and that gets trapped inside and looks bad. The easy way is to prevent it from getting dirty, by being tidy, and minimizing the amount of time that its open. In other words, work in a clean area, dont open it up until you are ready to complete the task, and close it back up. It also means being careful as you work, and get in and out quickly with little fuss.
If you have needle gauges, like my car has, you have to be careful not to bump them, or they will read the wrong position, I should have taken a picture before and after, just to make sure that everything was in the right place.

Step 3: Making the Wiring Cables

With a regular LED strip, you plug one strip into one controller, but Im going to be using LEDs in many places, all connected to one controller. One controller will control up to 5 meters(nearly 17 feet), a total of 300 RGB LEDs. Some strips and controllers will do more, some less, but 300/5meter seems like the most common.
The dash uses a couple feet, and the door lights use a few inches each, which leaves a whole lot of LEDs left to put where ever you like, as long as you get the LEDs to stay put, and connect the four wires.

I put LEDs under the doors because I want to be able to see the ground when I step out of my car, and making it the same color as the dash, or other LEDs is nice. I like mine on slow color cycle, rather than just one color, its too hard to pick a favorite. Running the LEDs in the door lights (both factory door lights, and the under door lights) require running 4 wires all the way to the inside of the car door. You dont need very thick wires, because its not delivering very much current, even to run 50 of them under the door. Its nicer if the wires are bundled in a sheath, like phone or ethernet cables, were they have many small conductors, in a single cable. I chose to use ethernet, because it was what I had handy at the length that I needed. The problems with most ethernet cables is that they are solid core wire, and the insulation shrinks a lot (its designed for crimping, not soldering.) If you might be able to do a nicer job, with crimp connectors, but I prefer solder.
The cable has to go to the far end of each door, which means that you need a fairly long section of wire, or at least 2 half length sections joined together in the middle, somewhere near the controller hopefully. I made connectors with inexpensive SIP pins/sockets, but you could get as fancy as you like, or even solder everything together with no connectors. The ethernet cable has twice as many wires as you need, you can use the other set wit ha different controller, if you want to get fancy, or you could use the second set as a back up for repair, or to double up the wiring in case you want to use more LEDs.

Step 4: Building the Polarity Inversion Circuit for the Door Lights

My door lights have a switch that connects to ground when switched on, to make this work with common anode wiring, I either need 3 door switches (one for Red, Green, Blue), or I need to make it switch +12v to the LEDs, when the ground is connected. With cars, you usually see a relay do this sort of thing, but with LEDs, its usually a MOSFET. This means the wiring is a little bit tricky, and to make things more stable, you need to put a resistor on the MOSFET, its called a pull up resistor, it makes sure its always off, unless we want to turn it on by connecting it to ground.

A MOSFET is a transistor, and for the scope of this discussion, basically what a transistor does, is it uses a little bit of electricity to switch a lot of electricity which is exactly what a relay does, only a transistor has polarity (like a diode). The polarity is important with transistors, if you need to switch to ground, you need a NPN/ or N type transistor, if you want to switch + DC voltage, then you need a PNP, or P type transistor. The way transistors work, if you want to switch to ground, you send a little bit of +voltage, if you want to swich + voltage, you need to connect to ground, which is what I mean about polarity inversion. With a transistor in between, when the door switch connects ground to the transistor, the transistor will connect +12v (from the door light connector) to the LED strip.

Generally, for low amounts of current you use BJT (BiPolar Junction) transistor, and for higher amounts of current you use MOSFET (Metal Oxide Semiconductor Field Effect Transistor). If I was just doing the door lights, and not the under door lights, a BJT transistor would probably be fine, but to be on the safe side, Im using a high current MOSFET, because im using a fair amount of LED, and I dont want to push the limits of the transistor.

Before you go and build this circuit, you should check how your doors work, if your car switches +12v, then you dont need this circuit at all, just hook the +12v to the LED strips common anode lead.

I include a couple images at the end of this document of how this circuit looks.

Step 5: Building the Door Lights

I wanted to replace the regular bulbs in the doors, with RGB LEDs. it turns out that was a lot of work for a little bit of light, but it looks real nice. Since I wanted RGB LEDs, I had to build a custom fixture to replace the white bulb. I used SMT LEDs, because they are small, thin, and fit inside the fixture easily.

To start, I made circuit boards that were small enough to fit inside the fixture, and big enough to make the most light. I managed to get 3, 3 LED segments (these strips use 3 LEDs at a time minimum to work at 12v.). In retrospect, 2 might have been better, because 3 caused some overlap, and shorting. I spent way too much time fussing with these little lights, mostly because I was trying to fit as many as I could, but it turns out that 6 LEDs would have been just fine.

One of the tricks that I used to make these was to strip a large bit if insulation off the end, then measure where I wanted to solder, and just cut the insulation, and slide slide the insulation to make a bare spot for soldering. This makes the wire a single wire, instead of several small wires, and that makes it a lot easier to solder.

Step 6: Under Door LEDs

These LEDs are sort of like underglow that you used to see on cars, but these only work when you open the doors. They look cool, but they are functional too, they do a real good job of illuminating the ground so you can see where you are stepping when you get out of your car.
You could add more LEDs than just under the door, putting some above the footwell, or under the seats would also be a good addition. I originally planned on adding some in the dome light, but after running cables to the doors, I decided that the dome light was going to be more effort than I was willing to do, but the dome light would also be a great place for more LEDs. Some other areas that might be good places for LEDs are trunk/hatch, under the hood or engine bay.
I havnt done it, but Ive also considered putting LEDs between body panels to light up all the panel gaps, and in the wheelwells and around the brakes, those will probably get you tickets if you drive with them on though.

With the cables run to the doorlights, and the MOSFET circuit in place, all that you need to do to add the under door lights is run 4 wires from the door lights to the underside of the door. My car has a hole with a rubber plug, but you could drill a hole or run the wires out from between the interior panel and door. You could put a grommet in the hole, and after you put your wires through, put a dab of RTV in the grommet to make it waterproof again.

You could put connectors on the LEDs and wires, but it would have to fit through the hole in the door, and I didnt want that, and I chose to solder my strips directly to the wires. After i hooked things up, I tested the lights to make sure everything worked right, then cleaned up the flux and solder mess.
After things are soldered and cleaned, I put a bit of tape on the car as insulation and cleaned the underside of the door really good with soap and water and green scratchy pad. Then I peeled the backing off the LED strip, and ran it along the bottom of the door, pressing it on real tight. I added drops of superglue on the edges about every 6 inches or so to help hold the strip in place (because the strips adhesive gets soft when it gets warm, and may not hold up to summer heat).
With everything working right, and the LEDs permanently fixed, I carefully put the excess wire back in the door, and put the rubber plug back in (it wasnt easy, but it went), then added some more tape to cover the wires. The last thing is to put the door card back in, and handles and the rest of the door parts.
While you are working on the electronics, you obviously want them turned off, the easy way is to unplug the battery or LED controller. now that things are finished, you can hook it back up and enjoy whatever color or color cycling you want.

Step 7: Overview and Afterthoughts

If I were doing this over again, I would only put 6 LEDs in each doorlight. Trying to get 9 LEDs caused a lot of problems, and was needlessly difficult.

There are lots of ways that you can do this sort of thing, but differently. One of the ways that would be even more awesome is it use individually addressable LEDS, and a microcontroller (arduino, ...) to make the LEDs do more interesting things. You could do rainbow color cycling on the door lights, or scrolling marque type thing, or many other types of animations. You could program the lights around each gauge individually, and if your microcontroller is tapped into your cars various sensors, you could make your temp gauge glow blue when cold, and red when hot, you could make your tachometer flash red when you get to redline, you could make your speedometer flash if you go too slow or too fast (may require adding a GPS module to monitor speed and location), or how about sequential turn signal indication in the dash? There is a lot of good functionality that you can build into custom dash lighting.

Using a relay instead of a MOSFET is probably a better way to do it for a car, because accidental shorts will instantly break a MOSFET, but a relay is a bit more rugged and durable. I did have problems, and went through a few transistors while having problems with one of the doorlights.
I also broke a few controllers because of shorts and not being careful enough with putting connectors on the right way, there are a lot of ways to break stuff if you are not careful, and even if you are careful, its easy to make a mistake.
Most of the LED lighting products seem to be 12v, and that means there are a lot of easy to install options for cars. There are many kinds of LED strips, the ones Im using here are the most common type probably, but you can apply this to other types of LEDs too. Another common voltage is 5v, particularly with the individually addressable type. One way to get a set of those going easy, is to use a DC-DC buck converter, to reduce the 12v to 5v.