I drive a 2014 GMC Sierra 2500HD, it came with a class V hitch and the trailer package....the trailer brake controller was already factory installed in the dash. Yesterday I purchased a very used 1999 Big Tex 70PI trailer with electric brakes. When I plug the trailer (with an adapter) into the trucks bumper, all my lights work (running, brake and turn signal lights). But the trailer brakes seem to be applied whenever the connection is made, as if I was stepping on the brake but I am not. When I disconnect the plug, the trailer will roll freely. Adjusting the "Gain" up or down does nothing. Does anyone know if this is an issue with my truck or the trailer? Is there wiring on my truck that still needs to be completed? Thank you everyone for reading my question, I hope someone can help me.