[markcee]

Do you have a buddy that can hook up the trailer with his/her truck and give it a go? As you've pulled before with this truck it's possible - even likely - that it's the trailer wiring, but this could isolate the problem to trailer vs. truck.

Assuming you have the GM version of an integrated TBC....Do you get any error messages on your dash display?

With my issue, I was getting a trailer wiring fault message and it ended up being a short on one of the magnet wires...insulation was rubbing on the inside of the hub and worn through.