Gm turn swithch problem?

2002 Chevy 2500 HD. brake lights stay on with trailer or without trailer. with out trailer all three brake lites are on, on the truck. with the trailer the left side stop/turn is on full time. right turn will work. turn on left signal and brake light left side stays on full. with out the trailer if you put the turn switch half way between the neutral and left turn position the brake lites go off and if you then step on the brakes the brakes and turn lites work correctly. if you pull the fuse for the left turn signal (trailer) the system works properly without the trailer. i have taken the electric brake control out of the system completely. removed all bulbs and checked for a jumped filament. no one at gm has mentioned any other brake/turn control other then the multifunction switch (turn swithch). i changed the turn switch and everything worked correctly for a week. $441.51 a week for lites to work is a little steep for me. i checked the trailer and it works fine on another vehicle. any thing that i may have missed?