There's no 12 Volts to the middle bulb socket with the brake lights on.
I measure 2.2K ohms from the ground leg of the socket to the car frame!
So it's time to look for a loose ground wire.

How does the trim above the back seat come out to expose the wires?
There's the center brake light, a pair of speakers and de-icer wires to the rear window all hidden under and I can't see them from the trunk.
Or maybe there's a ground wire junction somewhere in the trunk that could be sour?

The schematic is fuzzy about the middle brake light with an early and late version
that have different ground wire junctions it seems

Thanks, Bill