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2002 2500HD 6.6 LB7
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Discussion Starter · #1 ·
Bought a 02 2500HD, it was a commercial plow truck. Previous owner had made a custom mount to get rid of third brake light and mounted a light bar for reverse. it didn't work when i got the truck. took it off to see how it was wired and test wiring.

there's 2 sets of wires up there. the plug with the red and black wire, i cannot get to light up a test light at all. cargo light button, aux light button or brakes or reverse.

the second set i cannot get the green or blue with tracer to light up at all. orange is always hot and black is ground.

i would like to wire the light bar as a reverse light. not sure we're to go from here.
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2019 GMC Sierra 3500HD
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160 Posts
I’m guessing here…but one set is for the 3rd brake light the other is probably the bed light.
 

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Best thing to do is to get the electrical manual with wiring diagrams for your year and model of truck at Body Builder Manuals - GM Upfitter and that will tell you what the wires were originally designed to do. With a test light, you can then see if your truck has the correct inputs/outputs to your light. Once you know all that, you can modify as needed to get the desired lighting.
 

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2007 GMC Sierra LBZ
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2,479 Posts
If i remember correctly the switching happens on the ground side. It has constant power with the grounds being the controlled circuit.
 

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2007 GMC Sierra LBZ
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2,479 Posts
No only with the brake light and the cargo light button.
 

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2007 GMC Sierra LBZ
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I’m not sure on the color but yes you will have constant power on one wire. The switches control the ground completing the circuit. The grounds will be off and on not the power. Get what I’m saying? The controlled wire is the grounds.
 

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2002 2500HD 6.6 LB7
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8 Posts
Discussion Starter · #11 ·
figured out the plug with the red and black wire is wired to the aux button in the cab. other wires must've been for the stock light.

when i hooked the light bar up and turned the aux on it didn't seem to have much power at all. barely lit up the leds. haven't put a multimeter on it yet. but doesn't seem to have a full 12 volts.
 

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[QUOTE="First6.6Dirty, post: 12395649, member: 641611"
kind of a dumb question. how does that wire not drain the battery down if it's always hot? or does it just have such little draw it doesn't really affect the battery's.
[/QUOTE]
Sometimes electrical devices are alwlays grounded, then electricity is added to complete the circuit, and the device does what it was designed to do. Other devices always have power to them, and then to turn them on, you add the ground side of the circuit.
 
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