2006 GMC Yukon electrical problems

I didn't understand part of your description, but you have an electronic flasher. Those flash real fast when a bulb is burned out or there's a break in the wire going to one of them.

Do you have two bulbs on each side on the rear? Most manufacturers have gone to that with one being the brake light and the other being the signal light, then both of them have the smaller filaments for tail lights.

Part of what you described sounds like the two sockets are switched around, but that obviously wouldn't happen on its own. Part of the symptoms also sound like there's a broken ground circuit. There is a computer involved in running the tail lights, and they cause weird symptoms. The first thing I'd do is check for chewed-up wires on a trailer harness. If you have one that was installed with Scotch-Lok connectors, those don't seal out moisture and should never be used outside. Those are the type you slide wires into, then squeeze the flap to push in the connector that pierces the wires. If you find those, suspect a corroded wire.

The next thing would be to remove all the bulbs from one side so nothing can back-feed and cause misleading test results, then work on just the other side. I would also remove all but the brake light bulb. Start with voltage readings on that socket. If that bulb acts differently now that the other ones are removed, that again points to a wiring problem. If that brake light bulb doesn't work at all now, diagnose that circuit first.