I'm having this same issue. I have a 7 pin connector at the bumper and a 6 pin on the trailer. I bought the wire adapter for the brake controler that plugs directly into the fuse block under the dash. Everything works fine except for the elec brakes. I took the truck to a place that installs brake controllers to let them figure it out and all it did then was lock up the brakes when I steped on the brake. No matter which way I moved the adjustment wheels on the controler they still locked up and the controller always showed an open circut (--). I may have to look for the wires under the brake booster and try that....
Just wanted to let you know your not the only one with this issue....
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2004.5 Silverado 2500HD 4X4, LLY, Banks 6-gun/speed brake/intake, Dual bypass oil filter, 4" exhaust, 265/75R16 Bridgestone Dueler A/T Revo2, AirLift airbags, AMSOIL in everything!
84k miles/30NOV11
Yeasu 857D/ATAS-120A - "When all else fails..."