Could be similar to my problem w/ '94 S500 Cpe.
The CAA [closing assist actuator] was 'cracked' causing a vacuum leak such that when the door was slammed, the passenger side CA wouldn't work.
Details in Menu#28 on my page: here's a direct link:
http://www.k6jrf.com/MB_Close_Assist.html
BTW, that's how the CAA device "broke". . . people slamming the passenger door.