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With a multimeter, you should test for 12 volts at both sides of the brake switch, then at the rear of the car.
Do you have rear directionals that work? Do they have separate bulbs?
Some cars with single rear bulbs pass the brake light power through the directional switch.
Are all the bulb sockets in their three original positions?
Some people swap them around during troubleshooting - I did that in out '90 744 TI and nearly burned up the lenses!
Often, when I troubleshoot, I take a fused wire and run 12 volts directly to the device that does not work - then run tests back to the power source. Take a piece of wire plenty long enough and cut it about a foot from the end you want to connect to the battery. Add female spade connectors to the cut end and insert a spade fuse - you now have a fused wire. I carry several wires with different ends in my 544 for emergencies.
A single strand of one of the wires can trip a continuity test, and show voltage, but it cannot light a brake bulb.
This happened to a headlight on our '88 745 GLE - I stripped the wire back from the headlight and found a lot of black gunk about 1 foot from the bulb and only one copper strand. I had to replace the headlight wiring back to the firewall.
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