1999 Blazer. I've searched for Brake Light and Brake Lights and this didn't come up for me. The issue is that my left brake light doesn't activate unless the right blinker is on. I've replaced all 4 bulbs two months ago, and I checked them all again and the filaments are all good. We also just checked every other exterior bulb and made sure the turn signals and hazards lit up as they should. We checked the trailer wiring grounds as well. I believe I've narrowed my choices down to either a grounding issue or a multifunction switch.

We're talking to a trusted garage that we know deals with a lot of electrical issues for their opinion and experience tomorrow, but has anyone else ever had this issue? I can't think of any other reasons why a driver brake light would only work during a passenger turn signal.

 

Sounds like either a malfunctioning Multifunction Switch (turn signal stalk) or crossed wires somewhere in the harness back feeding current through the unused wiring... So I agree with your assessment, but you have to dig into it further to see...