Adding Third Brake Light to 1990 GMC 1/2 Ton Pickup

Updated 08/23/2012 | Published 08/21/2012

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2-to-3 Tail Light Converter for Towed Vehicles w/ Incandescent Tail Lights

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Question:

I would like to install a 3rd brake light/cargo light into the cab on my 1990 GMC extended cab 1/2 ton p/u. I think I can tap into the dome light for the cargo light, but need to find the source to tap in for the 3rd brake light. Do you have a recommendation?

asked by: Cecil W

Expert Reply:

Later model GM trucks had a lead in the tow package wiring located above the spare tire that had a light blue wire in it that provided power for a third brake lamp on a camper shell. I know that the late 1990s models trucks had this, but I do not know when they started doing this. It would be worth checking for a bundle of wires wrapped in wire loom above the spare tire.

If this wiring is not present, to add a third bake lamp to your 1990 GMC 1/2 ton you would need to tap into the turn signal circuits to get the brake light signal. Because the brake lamp and turn signals are combined into the same circuit, you would need a vehicle-to-vehicle tail light converter like part # 118158 to split out the brake lamp circuit, otherwise your 3rd stoplight would blink with the turn signals.

The converter has 3 inputs, a right turn, left turn and a ground. The easiest place to access the wiring would be the harness behind the tail light assemblies. Tap the green wire into the right turn, the yellow wire into the left and ground the white wire to the truck frame. (My testing indicated that only one of the wires need to be used, but using both would make for a more reliable connection.) The red wire exiting the output side of the converter would provide your brake light signal, independent of the turn signals. The green and yellow wires on the output side could be capped off.

expert reply by: Mike L