How To Find Cold Side Of Stop Light Switch On Vehicle For Brake Controller Installation

Updated 11/10/2025 | Published 11/12/2015

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Question:

Hi, where can I find the the cold side of brake switch to attach to the brake controller, what color is the wire and where is it located. Thanks

asked by: Alessandro

Expert Reply:

Normally you can find the brake light switch under the dashboard at the top of the brake pedal. There could be a number of wires attached to the switch so you can use a tester like part # PTW2993 to test the wires. Since the auto manufacturers can use different color wires it is always best to use a tester and check the wires for function and not color.

You would need to find the wire that only shows power when the brake pedal is pressed. That would be the wire that gets attached to the red wire from the brake controller.

If you want to let me know the year, make, and model of your vehicle I may be able to provide more information for you when installing a brake controller.

expert reply by: Jeffrey L

Aaron

5/10/2020

I have a reese brake controller, Brake-EVN proportional. I don't know where my cold side connection is on my 99 dodge ram 1500. To connect the red wire to the stop light switch.

Chris R.

5/13/2020

To find the right wire, you'll just need to use a circuit tester on the wiring behind the brake pedal. You're looking for the wire that ONLY gets power when stepping on the brake pedal (and no power with any other function). Unfortunately I wasn't able to track down what color this wire might be on your 1999 Ram 1500.

What F.

9/20/2020

What if you have air barkes? (I have a 2004 Ford

Jon G.

9/21/2020

@W I'm not super familiar with air brakes but from my understanding there should still be a cold brake light wire somewhere - you'll just have to use a circuit tester like what Jeff and Chris mentioned. If you absolutely cannot find one then installing something like the Roadmaster Universal Stop Light Switch Kit # RM-751000 might be your best solution.

5/14/2024

Trying to install a Roadmaster brake light relay in 2018 F150. I have two of four wires at brake light switch that give 12v when brake is depressed. Purple/white and purple/pink

Mike L.

5/29/2024

@Ed You're wanting to use the purple/white wire.

Kee

6/12/2024

I have 1996 xj Jeep Cherokee. I need to find which wire is for cold side of brake light switch? There are several lines coming out of the switch. Which color is it for cold side of brake light switch?

Bud M.

6/12/2024

@Kee I was able to find one source which said it is going to be a white wire with tan stripe. You should verify this using a test light on your particular 1996 Jeep Cherokee XJ to ensure it functions correctly when pressing the brake pedal.

Kee

6/12/2024

Thanks

Bud M.

6/12/2024

@Kee You're welcome!

Tim D.

8/3/2024

Hi Jeffery. I have a 2010 Chevy Express 3500. There are four wires coming off the wiring harness from the brake pedal. None of them seem to be a cold switch, at least with 12 volts. One of the wires is on when the pedal is not activated and then turns off when it is depressed. Do you know or have any suggestions of how to install my electric brake controller? I have a Takonsha IQ progressive brake controler.

Jesse M.

8/5/2024

@TimD These Chevy Express vans can be tricky but if your vehicle has the OEM 7-way tow package the blue output brake wire is already ran and it is located under the dash near the emergency brake pedal. Some models however will not have this wire or the 12v power wires run and in this case you will need to run your own wires using the # 5506 kit.

Tim D.

8/7/2024

@JesseM Thank you for the reply. My van did not come with a tow package. I installed wiring for trailer lights and then the ETBC7 wiring kit from e-trailer. I called to ask the same question, and the best answer I got was to connect it to a fuse. When I went to look for the fuse description they gave me, it does not exist on this van. I tried wiring it to the brake switch fuse, which doesn't seem to be the answer. Any advice? I'm sitting here with everything basically done except for a place to put this wire.

Gilberto

2/7/2025

I am hardwiring my tekonsha electric brake controller on 2021 VW Atlas that didn't have oem plug in,so I am having a hard time finding the cold side wire for the brake switch ? Any ideas where I can connect the red wire from the brake controller on this vehicle?,thanks.

Robin H.

2/8/2025

@Gilberto I'm sorry to say we don't have any specific information on the location of the wire. You will either have to test as described above or install the # RM-751000 which can be used to connect the red wire as well. Sometimes on newer vehicles the wire cannot be accessed but again, I don't know if that is true on your 2021 VW Atlas.

Gilberto

2/9/2025

@RobinH Thanks for the response, luckily I was able to connect the red wire to the brake switch located at the Brake master cylinder. An other thing that I noticed is when car is parked not running the brake lights don't wort but if switch is on or car running the do work like it should! Any suggestions? I already checked ground wires on the T- connector and the universal 7 way connector and everything seems to be right.Thanks.

Robin H.

2/9/2025

@Gilberto do the vehicle brake lights have power when it is parked and off? If they don't that explains it. Either way, I wouldn't worry about it as long as they're working when the car is running since that is when you need them.

Lonnie

5/14/2025

Trying to figure out which wire to tap the red from my brake controller into, I have a 2016 Jeep G.C. Laredo 4x4, when I unplug the wires going into switch, I see a green w/ yellow stripe and 2green w/ brown stripes, Is the green & yellow the cold side? Thanks

Robin H.

5/19/2025

@Lonnie it looks like the brake light switch is actually unusable on the 2016 Jeep GC so none of those wires will work. You will need to use the Roadmaster Stop Light Switch Kit # RM-751449 which will act as the cold side for your brake controller installation.

Paul

11/9/2025

Hi Jeffrey, I could use some help. I have a 4 pin to a 7 pin wire connector. As soon as I turn the tail lights on, it blows a fuse. My 4 pin works fine. What is your advise. Thank you, Paul

Kate F.

11/10/2025

@Paul It sounds like your 4-pin setup is working fine, but when you use the 4-to-7-pin adapter and turn on your tail lights, you blow a fuse. That almost always means there’s a short or wiring issue in the adapter or the trailer side, and it’s easy to narrow down. Start by checking how your adapter is wired. A standard 4-to-7 only uses four pins: ground, tail/running lights, left turn/brake, and right turn/brake, and leaves the rest empty. If it’s wired wrong or has an internal short, it’ll pop the fuse right away. Try plugging in just the adapter (no trailer) and turn on your lights. If it blows the fuse, the adapter itself is bad. Next, make sure the white ground wire from the adapter is connected securely to clean metal on the vehicle frame. A weak or missing ground is the most common cause of this problem. You can even test it by grounding the white wire directly to the negative battery terminal, if that fixes it, you’ve found the issue. If the fuse only blows with the trailer plugged in, the short is on the trailer. The most likely culprit is the brown running-light wire touching the frame or pinched somewhere along the line. Also, if your adapter has extra wires (like black, blue, or purple), leave those disconnected for now, only the four main lighting wires should be hooked up. What brand adapter are you using? Is it new, or have you had it a while and it now starting to act up?

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