[buzzinhalfdozen]

My son just had a similar issue with his rig...only when he turned his ignition on the trailer brakes would apply (turning the ignition on makes the Aux. battery terminal hot in the 7 pin plug, at least on the fords) He unhooked the plug a few times and it stopped but I'm certain there's corrosion in either the vehicle plug or the trailer plug or both. The only other option is one of the 2 is miswired. If the truck has the factory 7 pin plug then that would be wired correctly, at that point I'd look closely at the trailer wiring. Good luck Joe BTW any modern enclosed trailer has a dedicated ground wire in it's plug...so grounding thru the ball is a thing of the past.