Wiring Third Brake Light for Truck Cap on 2014 GMC Sierra 2500

Updated 07/24/2018 | Published 07/22/2018

Question:

I have a 2014 GMC Sierra 2500HD Old Body Style I trying to wire up the third brake light on the truck cap but cant seem to find the wire. The shop that I purchased the cap from installed it years ago said there was no way to do this because of the way GM has their wiring harness set up. Ive been reading some forums online and it seems as though this is possible with the newer body styles... 2015 and newer. Could someone please let me know if i can in fact wire this and how to do so. I do have mechanical knowledge so i could do this myself but i do not have access to the wiring diagram and would prefer not to prop each wire. Thanks in advance.

asked by: Jeremy

Expert Reply:

Based on my research, it looks like both the previous generation and later model years of the GMC Sierra featured easy to reach wiring for the dedicated brake light circuit on the truck. For your 2014 model Sierra though, these wiring harnesses are not present.

You do still have a couple possible options for getting a dedicated brake light only circuit for a third brake light on your truck cap. The easiest method might be to simply check your truck's owner's manual and locate the CHMSL fuse and tap into it there. This will give you a circuit that only powers with the brake lights activated.

Otherwise, since your Sierra has a combined wiring system, you can also just tap into the brake light circuit and then add a converter like # 118158 so that the third brake light doesn't also illuminate with your turn signals. The converter will take the combined signals and separate them. On the output side you will have one wire each for the turn signals which you won't need, and then one red wire that will be used for the brake light only.

expert reply by: Chris R