low voltage at trailer plug

[Henry L]

2020 Silverado 2500, have taken this truck to two different GM dealers and they cannot find an issue. At the at the 7-pin plug when the manual gain is depressed at 100%, it only puts out 7 volts. Need 12 to lock up brakes. Cannot properly set up my gain when hooked to trailer before towing. Dealership has replaced fuse block brake controller and I had them change out the plug on the bumper. Still the same. At neither dealership have they been able to give me voltage going in the controller and coming out. One even said they can't put a volt meter on it. I believe it's a wire somewhere. But due to my health issues I cannot physically work on my truck. I'm probably going to end up taking it to an auto electrician. Any thoughts!