I recently got a cap for my 2012 Toyota Tundra and am wiring in the brake lights on it. I was planning on pulling off of the existing brake light but there are 3 wires going into it and only 2 going into my truck cap brake light. There is a black, solid green, and green + red stripe wire and I am not sure which is which.

What are each of these wires and what do I need to hookup to get the brake light working?

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  • Welcome to Motor Vehicle Maintenance & Repair! Usually the black wire is the ground. This leaves you two to choose from. Back probe the other two with a multimeter to figure out which is which. The turn signal is usually easiest to figure out because you can leave that going while you're checking. Brake lights you gotta have someone else standing on the pedal or jerry-rig something so the brake light will stay on all the time. Commented Feb 16, 2021 at 2:02

2 Answers 2


Paulster is right, I'm just going to expand on it a bit. The light on your cap will only light up when the brake is pressed, that's why there's only two wires. One of the two wires is a ground (likely black). Connect this to the ground for your tail lights (also likely the black wire).

Then to address the two green wires, find out which wire gets power only when the brake peddle is pressed. Whichever that wire is, that is the one you want to hook up to your cap. The good news is, if you mix up the green wires it's very unlikely to hurt anything. If the cap light always stays lit, you got the wrong one. If it never lights up, the ground is wrong.


The method above will work for any brake light and is what I used to figure this out. But for anyone with the same 2012 Toyota Tundra who comes across this, connect the black wire to ground and the green and red wire to red on your cap's brake light.

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