Mercedes Benz repair: Mercedes 1988 300SE, interior panel, turning the key

Question
Turning the key in the drivers door lock meets very little resistance and doesn't lock/unlock door.  Using the trunk lock, all doors lock/unlock.  Attempting to unlock car using passenger door causes alarm to go off.  Removal of the drivers door interior panel I found, at the bottom of the door, a section of a small white plastic part (broken off). Please email any suggestions.  Thank you.

Answer
Call Phil, he knows exactly what part failed, he can give you the price and part number.

Passenger door causes alarm = the door lock is failing, ask Phil about this also.

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