How to Add a Circuit to Your Car




About: Hi, my name is Jan and I am a maker, I love building and creating things and I am also quite good at repairing stuff. Since I can think I've always loved creating new things and thats what i keep on doing ti...

Maybe you are also in a situation where you want some extra functionality in your car.

Be it an extra Output, a central door locking system or an integrated coffee maker.

In this instructable I want to show you how to properly add a new circuit to your car using an add-a-fuse-adapter.
You can also watch it as a Video on my new YouTube channel.

Safety Note: You should only use this method to add circuits that use less than 10 Ampere of current, because it partly uses your car's original wiring. For circuits that operate at higher current's, such as Audio amplifier's you need to run a seperated wire from the positive terminal of your battery to your device and do not forget to add a fuse near the power source.

Always use wires and switches with a suitable size and insulate your wiring to prevent short circuits.

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Step 1: Materials and Tools

For this project you will only need a few things.
The materials are the following:

  • an add-a-fuse adapter + a suitable fuse (There are different sizes. Use the size that your fuses have.)
  • some wire with the right cross-section (see down below)
  • isolation tape and zip ties
  • crimp connectors (ring, flat and wire to wire)
  • a switch that can handle the current your need or a smaller switch + a relay (magnetic switch)
  • your new device (a phone charger in my case)

And you will also need a basic set of tools including:

  • a multimeter
  • a pair of pliers and a wire cutter
  • a crimping tool
  • a screwdriver + different bits and nuts
  • plastic tools
  • a flashlight
  • a drill

When you picked everything up go to your car and start working.

Choosing the correct wire size is very important . If the wires are to thin they overheat and can cause a fire. A simple formula to determine the size you need is this one:

( I x L x 0,018 ) / U = A

I : the current that will flow through your cables

L : the total length of the wires in meters ( in this case from the fuse box to your grounding)

U : The maximum Voltage loss you want to have in your wiring ( I recommend 0,5 V)

A : the right cross section in mm^2 (for americans there are tables to find the AWG equivalent)

Step 2: Measuring Your Fuse Slots

The first thing you need to do is locate your fuse box. It can be under the hood or inside your dashboard or both, many modern cars have more than one fuse box. Your owners manual will tell you were it is.

Open it and use your fuse removal tool to remove one fuse at a time. Connect the negative wire of your multimeter to your cars body, set it to 20V DC and measure the voltage of both pins. First measure without the key plugged in.
If a slot's pin shows 12V this means 2 things:

  1. This slot does always have power. You would use such a slot for devices such as an alarm system or our charger that should be able to charge a phone while the car is parked.
  2. It is the input, the other pin is the output and should measure ~ 0V more on that in the next step.

Reinsert the fuse and mark the slot with a post-it or some tape.
If you need a slot that is powered when the key is in position 1 for devices like a radio, you search for a slot that has no power when the key is not inserted, but is powered when it's in position one. Mark it in a different colour if you are connecting more than one device.
If you want a device to only have power in key position two, you follow a similar procedure.
Keep on measuring until you found the right slot for your application.

Step 3: How Does an Add-a-fuse-adapter Work

I mentioned that it is important to know where the input and the output of the slot is. I'd like to explain the reason for that with this little animation .

Your fuse slot has two pins(1) the input is connected to the positive terminal of the battery (2) and the output goes to your original device(3). The battery and the device are also connected through your cars body, they are grounded.
The bottom slot of the add a fuse adapter is for your original fuse (5) that, as usual, completes the original circuit (6).
The top slot is for your new fuse. It connects the input or the positive terminal of the battery to your new device and closes your new the circuit(7).
If you plug the adapter in the wrong way (8), your new circuit won't work without the original fuse (9). As if this wasn't already bad enough, when both fuses are inserted the current for both circuits runs through the original fuse (10) , which will probably cause it to burn out (11).
Now you know why it is important to plug the adapter in the right way.

Step 4: Finding a Way for Your Wires (Corsa C Specific)

So you found all the slots and know a spot where you want to mount your device? Than it is time to run the wire in between the two. For this you will probably have to take a few parts of your car apart. Which parts you need to remove is of course very vehicle specific. This step will cover how to do it in my 2005 Corsa C.

On the outside:

  1. The first thing that needs to be removed is the wiper. It will probably be rust-welded to the thread and some force will be required to remove it. You simply use a wrench, that fits nicely around the thread and a Hammer (also good to relieve pressure, if you get frustrated). Protect your car with an old towel and carefully hammer onto the thread several times while pushing the wrench upwards. Eventually it will come off.
  2. Open the hood and unscrew the two screws that hold the water reflector in place. Squeeze it out and put it to the side. You won't be able to remove it completely, as the cleaning nozzles are still attached.
  3. Now you have access to the body control module's cover. Unscrew its' 7 screws and take it out. As you are already taking it out, inspect the sealing. It is probably corroded which is a common problem in the Corsa C and leads to water flowing into your footwell. You can replace it with some window sealing.

Inside the car:

  1. Remove the small storage compartment under the steering wheel by releasing the two noses on the top of it. This is pretty tricky.
  2. After the small compartment is removed you can unscrew the two screws that hold the big panel under the steering wheel in place and pull it out from the bottom.
  3. Pull the seats to the front and remove the 2 screws on the back of the center console, then pull them back again and remove the two screws on the front. You are now able to lift the center console up a little bit. This will allow you to stick tools or your hands under it from both sides.
  4. Also remove your radio with a suitable removal tool.

Step 5: Pulling the Wires Throug Your Car (slightly Corsa Specific)

You now have a way for your wires to go through. Unfortunately there is an obstacle. An almost fully packed rubber tubing. How are you going to pull the wire through this? Luckily there is also a trick to handle this. You can use a thick piece of garden wire and cover the tip with tape or shrink tubing but the better solution is to use a thinner wire, fold it 4 times and twist it.

Now you have a special tool with a round tip. Start pushing it through from one side of the tube while navigating it from the outside with your other hand. This is the trickiest part of the whole project and it might take some time, keep calm.

When you eventually reach the point where the tip comes out on the other side, hook your cable up to the and and secure it with tape. Use your pliers to pull it through and thread the wire into your car, so that it comes out in the footwell. From there you lead it to the radio slot from behind. Pull it out of the slot and push it back in downwards. Stick your hand under the middle console from the side to grab it and pull it out there. Thread it through the hole of the original 12V output and you are finished.

Step 6: Crimping the Connectors

This step is actually pretty straightforward. You remove the insulation of your cable, put it into the crimp connector on the adapter and compress it with your crimping tool or your pliers. You can also put some shrink tubing or isolation tape around the connection to prevent it from corroding.
Insert the fuses and plug the adapter in the right way.

Step 7: Connecting Your Device

Time to mount and connect you device, but before you can mount it you need to make a grounding. I crimped 3 wires into a cable shoe - not the best solution - but I also soldered it and added shrink tubing. I atached it to a screw under the middle console.

I made myself a nice little panel with several switches because I want to connect more than one new device. I also soldered wires to them and crimped flat connectors onto every cables end.

My switches do also have angel eye illumination and ,as I am mounting the panel where my original 12V output, which was also illuminated, was located, I will connect it to my original pin (shematic 1).

You can also connect the illumination to the switches middle pin or to the positive terminal of your device , so that it indicates when the device is turned on.

Wrap the connections with isolation tape and hide the wiring.
Test if everything works as it is supposed to and secure the wires.
Now reassemble your interior.

Step 8: Enjoy the Advanced Functionality

After you are done making your dream of an advanced car a reality, you can finally enjoy the advantages that it brings to you. Go for a ride and show your new gadget around.

I hope this instructable was helpful for your project and I would really like to know if you like the way it was presented to you. If you have any questions feel free to ask. As this was my first video project I am also curious to know what you think about the video. Feedback of any kind is appreciated. Good luck with your future projects and have a great day :).

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    13 Discussions


    1 year ago

    Thank you for showing me the Add-A-Fuse device. I had never seen them and they will make adding a power outlet ("cigarette lighter" socket) for my dash cam and backup camera in my truck much easier. I was expecting to have to trace wires and splice or use a tap connector like tidux suggested and even with a wiring diagram, it was going to be difficult.

    There is a number of wire size charts available online. I rarely ever use wire smaller than 18 just because the smaller sizes don't tolerate rough handling and often their insulation is very thin.


    2 years ago

    Great write up and video, I plan to do the same thing but I'll be using a control panel with a twin usb and 12v cigarette lighter chargers with no switches and power when in ignition position 1.
    My question is can i run 1 wire from the add a fuse to run both chargers on the control panel or do I need to splice the wire into 2 (1 for each), I'm just a little confused about your wiring to the unit.


    3 years ago

    OK thanks, I'm a bit paranoid about wire size vs amps in a car.

    1 reply

    It is good that you have some respect for the right wire size. There are way too many people who simply use any wire they can find. I use the formula to determine the minimum size i need and then choose a wire that is one or two sizes bigger because bigger is better in this case:D.


    3 years ago

    wel done deep up posting


    3 years ago

    A question, are you certain about the formula?

    If i input 10 amps, 1 meter and a 0.4 voltage drop, it result in 0.47mm square, = AWG20... kind of small for 10 amps!

    I used this chat for equivalence:

    1 reply

    The formula is correct. I know that it seems pretty small but you have to consider that the resistance of the copper wire is directly proportional to it's length. So if you double the length the required cross section to achieve the same voltage drop also doubles.


    3 years ago

    This is a nice ible. I have seen many people do things like this a skip the fuse. The fuse is the most important part.

    May I recommend instead of breaking the original line completely to use a product called "mid line tap connector" or "displacement connector". Both names are the same product as far as I can tell. I always called them piggy back connectors.

    This connector snaps over existing wire and then you insert your new circuit into the connector that runs to an in line fuse holder.

    1 reply
    Basement Engineeringtjdux

    Reply 3 years ago

    Yeah, I used this type of connectors to hook my parking aid up to my reverse light. They are great but in my case the original wire from the fuse box does not suit my purpose because it is only powered when the key is plugged in. Although I could have used the original grounding. I need a charger that can be turned on at any time to charge my phone on camping trips and in parking lot's while I am shopping.


    3 years ago

    Hey that hammer use near the windscreen is trouble. Just use wd40 or powerlube to soak into the threads for 10mins first. No risk to the windscreen or any components plus less stressful to you. Otherwise really nice instructable.

    1 reply

    I did actually not know that trick. It sounds great. I will definitely try it out next time before using raw force and putting my windshield in or other exterior parts in danger :).


    3 years ago

    nice instructable and you very quickly mentioned the need for correct size wires. please may I just add that the amperage of your new devices is critical in choosing the right wire as under rated wiring could result in a fire, especially in the case of a permanent live installation I.e phone charger. if anyone is in doubt about the rating of wire to use, please ask a specialist.
    sorry to feel the need to bring this to the attention of intelligent people but as a recovery truck driver I have seen too many fires caused by otherwise well intentioned d.i.y.ers

    1 reply
    Basement Engineeringsmaster2

    Reply 3 years ago

    Thank you for your comment.

    You are completely right, it is extremely important to choose the right wire size. I added a safety note and a formula to choose the right diameter.

    Unfortunately I also already had a few experiences in the past where I could see how powerful the current of a 12V battery can be. Luckily none of those happened inside a car or a building.