Trailer Brake Controller Issues 2001 Suburban

Installed new brake controller to wiring harness that plugs directly into panel below steering column. The problem is the controller does not activate brakes when I step on the brake pedal. But guess what, the brakes come on fast and furious when I turn on the lights. Have dealt with controller people and wiring from controller to harness is ok. Just got back from RV repair place and he's stumped. Even tried hooking up another controller with the same results. Suspect is the wiring on the Suburban but I don't know where to look. Has anyone out there had something like this? Anything I can look at.