GM / General Motors: 2000 Montana trailer wiring problem

Everything works fine for the truck concerning the lights and my problem
arises when I try to hook up a trailer plug to it. I have purchased the
correct trailer wiring harness that plugs into my wiring harness at the rear
of truck, everything wortks ok on the rear but when I test the wiring
harness with the tail lights on and I press on the brake pedal to test the
braking circuit the test light goes out. This is before I hook up the
trailer so my problem is in the truck. Can anyone tell me what is wrong.I
talked to a trailer wiring person and he thinks possibly the grounding wire
is not grounding properly. All other lights work such as tail lights on left
and right signals work OK , Even using the cable converter for the trailer
works ok everything except that dang braking circuit.