Just to "muddy up the waters" here a bit, I'm wondering if you are experience the typical age old problem of your door not really latching properly, completely shut, when you close it, but otherwise it seems to function well in all other respects. In other words you have to shut it really hard, or repeatedly to get it to fully latch? If that's the case, I believe that your problem is not in the door latch mechanism at all, that is to say, not the complex mechanism installed in the door (which is a total pia to remove) but rather it is the striker mounted in the door jamb. If it seems I have identified the issue you are dealing with, I suggest that you examine the striker carefully and compare it's appearance to others on your car, most preferably the other ones that close proplerly. I'm willing to bet that you'll find that the rubber lip on the lower edge is either well worn or even completely missing. If this is the case, that is the most likely culprit and is available still from MB at a "New and improved" price! They were once very reasonably priced but now sell here at Park Place MB for a bit over $100 and that's with a MBCA discount! These can be found fairly easily in local PnP's but I would recommend procurring the ones from the rear doors as there is usually much less wear and tear on the rear doors. Take along an impact driver with a, I believe, 5mm alan type socket to fit the impact driver. And a good size hammer! Be very careful to make sure the bolt socket is clean and that the alan bit seats securely, you do not want these to "round out". I do hope that this will help solve your dilema. I believe it will. Just some more trivia, the key lock portion is in the door handle and many good lock shops can handle re-keying replacement door handles