1994 Mercedes Benz C280 brake lights

Your next step is going to be tracing and testing wiring. I would start at the failed brake lamps and start checking for voltage. This will require a multimeter. Check for voltage at the bulb socket. If no voltage. (Make sure the brake pedal is depressed for this check). Then move to the Brake lamp switch. If you have voltage at the brake switch then your problem is more than likely wiring between the two components. Since you have exhausted componentry that can be replaced on the circuit it is time to check wiring for faults. Wires don't usually fail but when they do it usually creates some headaches.