Do you hear any noises whent he locks try to lock?
I had the same problem on my car, the passager rear door would not lock.
I took ta panal apart and saw that the vacuum tube was rubbing against a sharp part of the inside door. I took some shrink tubing and re-attached it and bound it with some duct tape to keep it from chaffing.
1997 Silver AMG C36
OEM Advantegarde Grille
OEM Clear Corners
W210 Blinking mirrors
Zender Rear Lip Spoiler