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.

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1997 Silver AMG C36
OEM Advantegarde Grille
OEM Clear Corners
W210 Blinking mirrors
Eibach/Bilstein Combo
Zender Rear Lip Spoiler