design faults
Check around the rear door latch for rust. The catch moves every time you use it, wears the paint off and then the crud drawn onto the back door starts it rusting. It's covered under the warranty, but insist the repairer seals it with flexible sealer where it mates up to the door or the same thing will happen again. The metallic paint seems to attract stone chips, even on the roof. Is a solid colour any better?
Sounds like you need to re-align your rear door and lock. Retighten all hinge bolts etc. The door if properly adjusted should not give problems, opening and shutting is or can be smooth and solid to M/benz traits.
Regards,
Sorry Ford Prefect,
My above reply was applicable to a G-Class, I have just been through the community forum and now, relise that your comment was for a M-Class, in which case there you go......
Regards,