Need help diagnosing brake light problem

I have a 99 Chevy Lumina. When I use my blinker to turn left, the passenger brake light will not work. It works all other times except when the left blinker is on. I took it to a mechanic who checked the ground, the bulb, the circuit, and the switches. The problem persists. Any thoughts on what could be causing the problem.

The multifunction (turn signal) switch is a good suspect.

He already took the old one off and put a new one in. It didn’t fix the problem.

Mystery light problems in my experience usually are bad ground connections.

My mechanic said he already checked the ground, cleaned up the bolt and still no luck on fixing the problem.

There are many paths to ground, check them all, if all els fails try replaceing the offending light assembly.