Also had same problem w/ S500 cpe. Had no air from vents except when RECIRCULATE was pushed.
Accidently found the problem. Decided to change charcoal prefilter, and after removing the plenum (behind the engine compartment), found that the linkage was not connected. Had to remove the plenum and take off the top (about six-eight screws).
The 'arm' from the actuator clips around a drive rod that opens/closes the vent door. It snaps shut around the drive rod but it was 'open'.
Maybe your problem is similar.
[Edited by JimF on 05-11-2001 at 12:35 PM]
Regards . . . . JimF
'94 S500 Cpe
Visit my Mercedes Web Page