How to Install Third Break Light for Cap on a 2018 GMC Sierra

Updated 10/29/2018 | Published 10/28/2018

Question:

I have not been able to locate the wire you mention in the 2017 models. My truck is a 2018 GMC Sierra, I took the spare tire out and I can’t find the blue wire shown in your picture for a truck cap. I am hoping for suggestions on where to locate it that you may have.

asked by: Scott R

Expert Reply:

The pictures you are referring to are actually for older Silverado and Sierra pickups. I am not aware of any special wiring near the rear of your 2018 GMC Sierra 2500 but you can easily isolate the brake signal by using the 3rd Brake Light Logic Module # PP20-702.

This connects to your left and right turn circuits and then gives you a separate wire for the 3rd brake light (see attached wiring diagram).

expert reply by: Jon G