The lock on the passenger side front door does not go up or down with the other doors, although it will move a little. If I pull or push (hard!) on it then it will lock or unlock and will make the other locks lock or unlock as well. From the outside I can't even lock or unlock the car with the key from that side.
Any thoughts?

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'88 300E (200,000 miles!)

You don't say when this started, but I am assuming it is associated with the cold weather. From your profile, I see you are in Louisville, so this makes sense as well. Since the other locks move when you are successful in moving the stubborn lock, the pneumatic system must be intact, though it may be possible the pneumatic actuator is gunked up. Maybe another member can comment on this possibility. I am assuming that the grease on the linkage inside the door frame has dried up. Removing the door panel, disassembing the linkage, cleaning, and then lubing with a good white lithium grease like used on door hinges should take care of your problem. You need a grease that doesn't thicken in the cold and has enogh body to it to stay adhered to the movement. It should last another 200,000 miles!

Hope this helps.

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Deezel
87 300TDT
150,000 miles