Rear Harness White Wire

Main harness under bed, driver’s side, I have 1 White wire that is hot when brake is pressed. This harness has the Brown, Yellow, Light Green and Dark Green(also a heavier gauge Orange and Blue that isn’t being used) along with this White wire. It was connected to the mess in pictures below. What it was connected to was definitely something aftermarket, but the White wire is part of original harness. The plug you see was up in the driver’s side taillight area and plug went up into area where rear stake hole cavity is. Got rid of it all. All light function is normal, but can’t figure out what this 1 White wire is for.

Should be ground for the trailer connector that's what it was on all of mine. If it has 12v+ it may be shorted or someone repurposed it. The blue wire is trailer brake and orange should be 12v+ all the time.

The white wire is for an auxiliary "center high mounted stop light". CHMSL i think thats what it stands for. That's why it is hot when brake is activated.

It is NOT a ground wire.

GMCTruck said:

The white wire is for an auxiliary "center high mounted stop light". CHMSL i think thats what it stands for. That's why it is hot when brake is activated.

It is NOT a ground wire.

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I saw this same answer in another GM thread and you are correct it definitely is a HOT wire. I just capped it off and tucked it into wire loom.
Maybe that’s what that plug was added for, an additional high mounted brake light on a trailer. Just guessing!

Thanks GMCTruck, you put me on the right track. Just read that some came with this wire to connect to either a camper or rear shell brake light.

88GMCtruck said:

Here is what is at the back of your truck, usually up behind the bumper attached to the bed, near the spare tire. It'll be a bundle of wires taped/tied together.

You must be registered for see images attach

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Thats what I went off of.

HotrodZ06 said:

Thats what I went off of.

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And that makes some sense too. But, in a few other forums this is discussed and it’s just like GMCTruck’s answer-CHMSL. The White wire definitely is hot and activated by brake. Also, PO had run a wire from there up into stake cavity, which probably means he had a cap on truck with a high center brake light. Where did that ^^^ schematic come from?

Most likely. The post I quoted from https://www.gmt400.com/threads/anyo...-rear-lights-trailer-wires.36047/#post-931764

I have a new wiring harness for my backup lights tail lights all of that I just with the four-way plug I need to know what wires go into a pigtail coming from the truck to the tail lights what where I need to plug in my wires I have two green one dark one light a yellow and a brown so I need to know which ones to put into the 4 plug to hook up to all my lights which one goes where

kattlove71 said:

I have a new wiring harness for my backup lights tail lights all of that I just with the four-way plug I need to know what wires go into a pigtail coming from the truck to the tail lights what where I need to plug in my wires I have two green one dark one light a yellow and a brown so I need to know which ones to put into the 4 plug to hook up to all my lights which one goes where

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There are factory service manuals complete with wiring diagrams available for free download (in the engine section)

But this is a pretty basic one.

Dark green - right turn/brake
Light green - reverse
Yellow - left turn/brake
Brown - tail, tag lights

Richard