It is possible to remove the endpiece from the door without the key. Copy and pasted from an MB mailing list:
"Can't remember precisely what was involved, but iirc the issue is that to remove it normally, with the key, turning the key for cylinder removal both disengages the lock cylinder from the handle itself and from the lever that's inside the door that you actually pull with the tailpiece on the end of the cylinder in order to operate the door latch mechanism. Following the procedure in the fsm it is possible to loosen and maybe even completely remove the 3 screws that secure the handle to the bearing bracket without first removing the cylinder. Because you can't turn the key, it's necessary to rotate the door latch operating mechanism from the inside in order to disengage the tailpiece from the mechanism's operating lever, so obviously you've got to remove the liner and detach the mechanism from the inside. What isn't shown clearly in 72-250 in the fsm is the little electrical pigtail that I think is part of the alarm system - the wire that Sixto mentioned having broken. As I recall that thing tends to tangle things up somehow as you try to rotate the mechanism such that I had to remove the end of the pigtail completely from it's plug in the door and finagle it some to free things up. That's about all that I can recall. If you can determine that the pigtail isn't needed I suppose you could just cut it intentionally, but for some reason that I don't remember I either didn't want to do that or the possibility didn't occur to me at the time. I just remember that it was a bit of a struggle to get it out of the way"
Once you have the endpiece removed, I suggest doing one of the following:
Option A (easy): Install a "keyhole delete" piece on the front passenger door, and swap that endpiece over to the driver's door.
Option B (harder): Remove the cylinder from the endpiece and have it rekeyed with a donor part. I'd leave this to a skilled locksmith, because getting the wafers in place and lined up can be a real chore.