Been having this issue with my 09 3500hd for a while now. I have replaced the brake controller relay along the frame rail and the fuse in the engine compartment a few times. It would fix the problem for a little while then it would start to happen again. Now a new one doesn't do anything. I have checked resistance from the trailer plug to the ground wire under the drivers door which im pretty sure is the trailer controller ground and there is no resistance. I have removed the wire loom from most of the harness in the back section of the truck and do not see any issues. When I probe the back of the trailer brake with in the cab while manually engaging the slider I do go from zero voltage to around 5 volts on 2 wires I believe. then there are another 2 wires that have voltage and when I engage the slider they go to zero volts. So I think the switch is working. I do have power getting to both the relay and module along the frame rail. The blue wire from the relay to the trailer connector does have continuity. But I don't know what wires from the cab to the module or relays I should text for voltage. Anyone have any ideas or resolutions????? Im stumped!