I went through all the topics here plus YouTube and fixed my brake light problem. I assume you have eliminated the bulbs and circuit boards at bulbs as problem, as well as the fuses. 1) I removed the instrument cluster. (I can do this in two minutes....because I had speedometer problems a few years ago and got lots of practice.)
2.) With strong flashlight, I lifted large red bulb sensor relay, without disconnecting it, and checked voltage with my DVMeter on terminal 54S that is the wire directly from brake pedal switch. I got 12 volts each time I pressed pedal. But, I got no volts when I checked terminals 54L and 54R that are the wires to the brake lights. Therefore, brake pedal switch is ok and bulb sensor relay is bad, at least on the part that does the brake lights.
3.) I cut the 54S, L, and R wires, close up to the bulb sensor red relay, and then stripped the three loose wires and put an orange wire nut on them.
4.) Brake lights work!!!
5.) Because I have no other lights out, I left the relay in place and removed the indicator lamp for lights-failure on the back of the instrument cluster to avoid aggravation.
6.) I had a beer and smiled.
Caveat - I do things like this all the time. I am a tinkerer. I don't recommend you do it. ha.

 

Forgot a wire...... The middle of the hatch third brake light on my 1990 245 is a wire I forgot. I just went back to the red bulb sensing relay and cut off the blue with black stripe wire on terminal 5 and added that wire to wire nut with other three wires. All three lights work with brake pedal now.