All the door locks and fuel filler flap operate in unison by vacuum. Your car should have a vacuum pump in the trunk next to the spare tire on the passenger side. When you turn the key in either of the front door locks, you should be able to faintly hear this pump run (a quiet garage helps, as does removing the foam insulation from around the pump). If this vacuum pump isn't running, may be a fuse or electrical connection. If you hear the pump running, then the problem would point to a leak, disconnected vacuum hose, or bad element somewhere. There is a good diagram on the 126 CD showing the run, but if I recall correctly,two lines leave the pump-one going to the trunk element, the other going to the filler flap then working around the doors. My filler flap element went bad and no vacuum travelled beyond. If your pump runs and no doors lock, you might check the flap first, then the rear passenger door, and so on. Toughest part of this repair will be finding the problem, the repair part is pretty straight forward if you have the CD manuals from Fast Lane. Good luck

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Earl McLain
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)