I'd immediately suspect the red "Bulb Out" relay in the fusebox. The single power lead from the brake light switch comes here and splits into two circuits. The usual failure is a burnt solder joint on the top deck of the circuit boards inside it. I've fixed a few this way, always the same symptoms. This is the highest-current connection in the unit apparently.
A replacement relay or a bit of work with the soldering iron will fix it.
Try a junkyard! I haven't verified that a 240 relay is the same.... they look like it. I guess with both in the driveway I ought to check that someday.
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Rob Bareiss, New London CT ::: '86 244DL- 215K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K

did you check with a test light to see if you have power coming to the rear light assemblies.if you dont have power coming to the light assemblies. you might have a faulty bulb warning relay. all of the rear lighting goes through the relay before coming back to the rear of the car. if you check and have power, maybe the circuit boards , the plastic flexible wiring boards on the light assemblies are bad, check very closely where the wires connect to the boards. sometimes the connection point is damaged or dirty and corroded.