BIZARRE!!!!! I wired Tow-pkge and trailer last week. The Running-lights are 12v(on-off). The Signals-lights activated by 8v-12v impulses. The Brake is the same when applied. The trailer will get ground from the plug. Okay, still with me????
Therefore since the brakes-lights on the trailer work, the Pedal-Switch is working fine. Therefore 12v is on/off at rear of vehicle. Remove lens cover and ground one tail-light yourself. Have someone apply brake. That will give you answer as far as ground is concerned. Get back at me and I'll look further into possibilities if you can't solve it.