Well, I just tried working with it a little more by lubricating it and now it doesn't work at all. I can't lock it or unlock it using the driver's door, but the trunk and passenger door still lock and unlock the doors perfectly fine.

The 190's (W201) have a metal rod between the key lock & the door latch.

Very common for those to break OR for the retaining pin to fall out. Removal of the key lock should provide you with info on your problem.

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