For a few months, I have been unable to lock all four doors using the driver's side door lock from outside with the key. When I tried to use the key from the driver's side, the black knob wouldn't go down, no matter how hard I turned it. But when I locked it from the passenger side, all four knobs would go down with ease. I could unlock it from either the driver's side or the passenger side.

So, I replaced the driver's side tumbler. Now, when I try to lock it from the driver's side, the black knob will go down but only on the driver's side. I have to go to the passenger side to get all four doors to lock. Any ideas?

Also, on the inside of the driver's side door, there are three black caps--one that covers the Allen screw that loosens the door handle and allows the tumbler to slide out, one that houses the anti-theft canon plug and the third looks like a white plastic Phillips head slot. Anyone know what this slot is for? It looks like it moves a latch inside the door, but I'm not sure why it's there.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles