-This instructable requires some knowledge of electronics and auto body assembly.
-This might involve taking apart large sections of your car's console, if this could void a warranty, do so at your own risk.
-In some states and municipalities, it is illegal to have non-signal lights on while your vehicle in motion. Check with your local authorities before using on the street.
Time: 3-6 Hours
Cost: $20-50 (depending mainly on how many lights you get)
Difficulty: Decently easy
Step 1: Gather materials
-One or several waterproof, 12V LED strings - $10 ea.
-Lots of wire. Two-strand polarized wire is great (red-black or white-black)
-SPST switch - Free-$3 (I got a lit-when-on one)
-555 timer - $0.10-$1.50 if you went to Shady-o Rack
-10000-15000 uF worth of 10v capacitors (you can string them in parallel to add the capacitance)
-15kOhm resistor - the bigger the resistor the slower the pulse - alternatively you can use a potentiometer
-Project box of some sort
-Scrap thin medium-density fiberboard (MDF) or sheet metal
-Soldering iron and solder
-Hot glue gun
-Various size drill bits
And of course a car. It's a good idea to know your way around the inside of your car a bit before beginning.
Step 2: Take apart your console
Then, since you're probably already under the steering column, look for an unused plug in the firewall, right by the fuse box on most models. This is for running extra wiring (like we're doing) and, once you pop out the plug, should lead to somewhere near the left front wheel well. Save the plug, you'll need that later.
Step 3: Add an auxiliary extension
Step 4: Install the switch
Step 5: Affix LED strands to the bottom of your car
Make yourself some 2" squares of MDF, two per strand, and epoxy one to each end of each strand. Let that cure.
In the meantime, you can pick out suitable spots to place the strings under your car. Anything that is directly part of the chassis is great. Be careful not to put it too close to the engine or exhaust line! If you want a more temporary attachment, you can bolt the MDF to it or hot glue it for semi-permanent. A liberal amount of hot glue seems to bond the MDF to the metal really well; I've driven 100 miles with my setup and it hasn't fallen off yet. I wouldn't recommend using epoxy or J-B weld in case you have to take it off for whatever reason.
Step 6: Wire the LED strands
Since every car is different, you're going to have to come up with your own way to snake some double-strand wire from the cabin to the bottom of your car. Generally though, there should be an opening straight to the bottom near the front left wheel well (for countries that drive on the right, it'd be the right wheel if you're across the pond :-P). Tie a loop of string around the end of the wire, then feed about a foot of the wire through the hole. You should be able to see it when you look under the hood.
Take a coat hanger, or perhaps using your hand, grab the end of the wire from underneath your car and pull it through. Duct tape it to the bottom of your car to keep it from going anywhere (you can take off the loop now).
Run wire from each of the LED strands and wire them all in parallel (red to red, black to black) and solder the whole mess together. I'd recommend using liberal amounts of electrical tape as well. Once again, stay away from hot components.
Make sure to leave 1-2 feet of wire inside the cabin.
Step 7: Circuit time!
The idea is the 555 timer generates a square wave which charges up the capacitors in the on phase. In the off phase, the capacitors light the LEDs, depleting over time and creating the fade out effect. Theoretically, during the on phase, the current used to charge the capacitors should detract from the LEDs, creating a fade in effect, but often the current is so large that this is nearly instantaneous. Alternatively, you can put in a switch that bypasses the pulse circuit (so that the ground from the LEDs goes straight to the mains ground) so you can go from pulse to steady on.
I more or less used capacitors and resistors that were lying around. You can customize the frequency using the 555 timer calculator. Alternatively, you could sub in a potentiometer for RB to get a variable rate.
The 12000 uF capacitor is just a bunch of large 12V capacitors wired together. You may need more or less depending on the number and rating of the LED strings to get the right effect.
Test out the circuit on a breadboard connected to the car's electronics before putting it onto a PCB. And then test it when you've soldered it before boxing it up.
Step 8: Installing the project box
I put two screws on either ends for the power, ground, power for lights, and ground for lights, so I could just screw on the wires. Make sure to leave enough on the inside to account for the twisting (and start with the screws nearly fully tightened when you solder).
Find a nice comfy spot somewhere up under the console to put it. Screw down each of the wires to their respective contact, and tape the box securely so it won't rattle around.