Here is the scoop.. '02 1500HD crew cab short bed.... just installed an LED light bar to the top of my Aluminum flatbed. Turn signals and brake light works just fine. I have one more wire to connect to the back up lights but I would rather connect to the cargo lamp. I tried the orange and the grey wire at the cargo lamps and they are hot constant even when the lamps are out and the cab switch is off. Ok, so I pulled the switch and tried all 5 of the wires there. Grey is constant power, Orange is constant power, Purple is no power, Black is a ground, and Blue with White stripe is hot with switch off but no power with switch on. IS there a way to get the LED to come on/off with the factory switch?
My theory is that the cargo lamp switch allows a ground through the Body Control Module. If that is the case, can I tap into a wire there? Where is that anyway?
Thanks in advance....

 

The cargo lamp is a ground searching circuit.

Allan

 

Ok, a little more work with the test light and a friend with a lot of experience installing relays and I have a solution. Because the new style GM's use the body control module to "turn on" the grounds, you need to run a relay that is also ground or Negative triggered.
To do it on my rig I used a regular Bosh relay and put constant power to the #30 and #86 terminals. This came from tapping into the load side of a 12v switched fuse in the power center in the driver's end of the dash. Because the LED draws so little amperage I could use one wire split to both terminals and the circuit is still protected by the fuse. (If I went to a 12v power source that was not fused I would put a fuse inline close to the source.)
On the output side of the relay I connected the LED Positive to the #85 terminal. The LED grounds to the chassis so when power is applied it completes it's circuit there.
I then ran a wire from the #87 Terminal to the Blue wire with white stripe on the rear of the cargo light switch. This wire is the one that turns to a ground by the BCM.
I am very careful with my wiring so after I verified that the relay functioned as needed, I soldered each splice and coated with 2 coats of Liquid Electrical Tape before tucking everything back into the dash.
In the end this was a very easy mod. The issue I had was thinking that the Blue/White wire carries power until the BCM completes the circuit. The test light provided a ground fooling me into believing it was another constant hot. Now I have a light over the headache rack that allows lots of light when I want it but doesn't reflect off the back glass when reversing, the issue I was trying to avoid all along.
I hope this helps so others with their lighting issues....

 

I didn't replace the whole 3rd brake light/cargo light unit, I replaced the individual bulbs with LED Bulbs and the cargo lights worked for a while then stopped. The 3rd brake light works fine. Also replaced my interior dome light and map lights with LEDs and I can't turn on the map lights with the push button switch only the roller switch for all interior lights are connected in some way as to why my cargo lights don't work?