@003 H2 Hummer here. Seem to have some trouble with 7 pin plug.

First problem encountered was when I picked up a cargo trailer mid day and connected to Hummer I got NO lights at all. No turn, brake, backup, etc.

I was not equipped to troubleshoot it there, so I took the short ride home as is. At home had other things to do so did not get to Hummer till 7PM (almost dusk). Guess what? The running lights and right side turn/brake lights work fine now.

However, left side of trailer only has running lights, no brake or turn. Checked the LT TRLR fuse on left side of instrument panel, good 13.5V on both sides. Checked the corresponding pin on 7 pin connector and got nothing. Had helper hold down brake pedal while checking. Hummer brake lights are good, and right trailer brake lights come on. But not left, and the 7 pin connector for the lights got no voltage. Also checked electric brake pin on connector, no voltage there either. And obviously, when checking these pins on connector, the trailer was unplugged.

Any advice?

 

There are many mysteries surrounding trailers and pin outs and many places for opens...

Best to start at your bumper trailer wireing harness interface. Check both sides of it with a dc set voltmeter.
If that all verifies good.

Take it to the trailer side and use your ohms meter to check each section and bulb for continuity.

 

Today I connected a battery charger to the ground side of the trailer plug and then tested each of the other pin functions. No backup lights were ever installed on the trailer so those did not light. Brake, left, right, and running lights all worked fine on trailer. Still not sure about electric brakes though. When I connected the 12V+ to the electric brake lead, I did not detect 12V across the magnet wires. So now I need to trace those wires to see where the open is.

As for the vehicle side plug, I do not get a left side turn and brake connection coming from the Hummer. Fuse is fine, so now I need to trace THAT out also.

 

Concentrate on the hummer side 1st. Until you truck is all good to the plug harness you will lose focus on the trailer side of the problem. Test first on the back side of the harness then the front with a probe conneted to a meter. Trace that back if needed. If you have no blow fuses. Look for a open condition.

Good luck!