The front blinkers work BUT BOTH LEFT AND RIGHT BACK BLINKERS DO NOT.

Sounds like Mercedes' answer was to just throw some money at the problem. First, be certain you're using the correct bulbs. When you say you took the tail lights out, do you mean the bulbs or the whole assembly? Using a voltmeter or multimeter (or a simple 12 volt test light), check for blinking power at the bulb sockets. If none, you'll need to trace the wiring to find where the voltage stops. Oh, and one other thought: Mercedes sometimes uses separate fuses for front and back lights, so check all your fuses. I hope I have helped you.