Anyone here know things about the GMC's electrical hook ups for trailers? I found this site helpful:
...but it didn't solve my problem. The boat trailer's got a surge brake actuator with a solenoid that kicks the brakes off for backing up. The solenoid is supposed to activate when the reverse lights are on. From what I can find, the reverse lights are controlled by the middle pin on my 7 pin harness. The problem is that although the truck reverse lights work, there is no power to the center pin on the truck's harness. I also went underneath to verify that the center pin is hooked up to the light green wire (per the etrailer site's instructions).
Then, I tested running lights and turn signals with the multimeter - both good, but reverse had nothing. Further, I checked the fuse under the hood and it was good - even swapped it with another to make sure. I'm running out of ideas! It's a 2007 Sierra, if anyone has any advice on how to further troubleshoot this thing.