I had a 4 wire harness, I replaces it with a 4 & 7 harness. It was working fine, now if I have the trailer wiring plugged in and I hit the breaks the running lights come on (not the DRL, the amber) also my electronic break control stopped working. I was chasing wires all night and I'm about ready to bring it to a pro have him strip the wiring out and redo it. Any ideas?

 

I would suspect a bad ground on the harness (usuallya white wire)at the truck, or on the trailer at one of the lights. when you hit the brakes, the power goes up the harness, and can't return through the ground wire, so it goes back across the park light filament in the bulb, and down the park light wire (the brown one on your 4 pin plug). From the plug it gets distributed into your vehicles park light circuit. If the problem happens with your 4 pin, and your 7 pin then its probably the truck that has the ground problem. If it only happens with one of the 2 trailers, then thats where your fault probably lies.
Good Luck
Julian