This is a common problem. There is a small shaft at the end of the lock cylinder that engages the linkage inside the door. The shaft is made of a poor grade of die cast alloy. It breaks, and it cannot be repaired. The bad news is that you have to replace the entire lock cylinder assembly. The good news is that it is an incredibly easy job and does not involve removing the door panel. With just a bit of work, you can swap all the old tumbler plates (nomenclature?) into the new cylinder and keep your old key. I think you can order a cylinder keyed to your VIN, but I'm not certain of that.

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1987 W201 190D