I'm working on adding a 7 pin trailer hitch to an existing 4 pin on a 2017 Hyundai Santa Fe.

The last step of the process is connecting to the brake switch wire that is wedged way up on the brake pedal.


On my car that bunch of wires has a sheath tightly wrapped around it along with some electrical tape. I'm a big dude so I'm wondering is there a easier way to get to that particular wire, either inside the engine bay or a fuse tap. As it sits I'm going to have to carefully cut into the sheath and then find the correct wire. I think it is also referred to as the cold brake switch wire.

I also wonder what this wire outputs as the break pedal is pressed. Does it have something as simple as a different values that relate to how far the pedal is pressed like a potentiometer? Or does it just signal that the brakes have been pressed 0v and 12v. If the latter, how does the brake controller know how hard to hit the brakes on the trailer?

  • Welcome to Motor Vehicle Maintenance & Repair! Have you thought about running it directly from the brake lights? Commented Aug 22, 2018 at 21:01
  • That was my thought as well. I haven't gotten out the multi-meter and tested that line, but 1) Will that give me how much the brake is pressed in 2) I think on newer cars the light is actually controlled by a different way and the line to the brakes has a constant 12v going to it, I think so that the car's computer can tell if the light is out or not.
    – rball
    Commented Aug 22, 2018 at 21:05

1 Answer 1


It depends on how your 7-pin is hooked up, but it looks like there's two common situations:

  1. If you just need to actuate the brake lights, it's probably far easier to tap into that wire at or near the car's brake light itself than at the brake pedal switch or anywhere under the dash. (as mentioned in earlier comments).

    This will just provide 0 or 12v direct to the brake lights on the trailer, and is usually used where the trailer does not have it's own brakes.

  2. If you are using an actual brake controller, that should output some kind of variable voltage, which will apply the correct amount of force to the electric trailer brakes.

    What makes it work? There are different types but I believe one of the simpler ideas just uses an accelerometer. If it detects that the vehicle is slowing down, it outputs voltage, which engages the electric brakes.

    I would assume that there is some secondary output from the trailer brakes that hooks up to the actual trailer brake lights, so they go on and off as well.

  • Awesome answer thanks. You're correct, I looked it up and it does use an accelerometer in conjunction with if the brake is pressed, it doesn't care how far it is pressed just that it is. Then on the brake controller you adjust the level of braking. Very cool to know, although I sure wish I knew that before I ran all the lines under the car.
    – rball
    Commented Aug 23, 2018 at 5:25

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