I can no longer turn the key in the driver's side door to unlock the doors for my Euro spec E250. I can turn the key to the right to lock the doors but I cannot turn the key to the left to unlock. I think the tumbler finally gave out--any other ideas besides a bad tumbler? Can I just do a swap with the passenger side door? Would this be one of those "simple in concept, but a PITA for execution" job? Or do I just have Mercedes order a new tumbler/lock?

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1993 E250 2.5L TD 230,000 kilometers
Vicenza, Italy