Wiring 3rd Brake Light on Truck Bed Cap for 2015 Chevy Silverado 1500

Question:

I bought a cap for my truck and I cant find any information on how to wire the brake light on the cap or what parts I need. I would also like to purchase and install an interior light bar inside the cap. The cap has 2 wires - 1 blue, 1 black. Can you help?

asked by: Mike S

Expert Reply:

If you follow wiring harness feeding the 7-way trailer connector from the bumper up toward the spare tire and peel back the wire loom, you should be able to find an additional blue wire in the harness. The blue wire carries the 3rd brake lamp signal for your bed topper.

For the interior dome light, you can get 12 volt power from the black wire feeding the trailer connector. You'd want to install a switch on the circuit like part # PK34513.

One of the wires would be power, the other ground. The black wire is most likely the ground, but you could test it by applying 12 volts to the blue wire and grounding the black one. You could also call the topper manufacturer, and they'd be able to tell you.

expert reply by: Mike L

Mike H.

5/25/2020

When looking for this blue wire do you have a picture of where it would be? I see the trailer brake blue wire, i'm not seeing another blue wire.

Chris R.

5/28/2020

Unfortunately I don't have a picture available of that blue wire. It'll be a bit further up the truck than the blue brake output wire you found. If you can't find anything - I would just start testing different wires in this area for function (it's possible it's just a different color on your truck). See which one gets power with the brake lights on.