GM-GMC: tail lights, side marker, ground wires

Question
I have chased all the ground wires for the whole system and yet I still have a problem with my drivers side tail lights going out when I apply my brakes. The running lights work, but go out when I step on the brake. My drivers side turn signal works no matter if I am stepping on the brake or not. When I turn the passenger side turn signal on and step on the brake the drivers side running light goes out??? I've replaced all the bulbs, including the side front side marker lights, and back up lights. I am at my wits end, please advise on this ghostly problem.
Thanks
Cary
Oh, it's a 1990 Chevy 1/2 ton ext-cab.

Answer
The symptoms indicate a wiring problem.
Either trailer light wiring was installed improperly or some of the original wiring in a harness has experienced some damage and wires have shorted together.
You will need a shop manual to provide the wiring diagrams and will have to probably trace each wire in the affected circuits to find the problem.
It will be a time-consuming and difficult problem to cure.