Tref,
Looked at the vacuum section of the MB manual I have for my 1979 240D. There has been some indication that things don't change too much between model years.

It shows a green-yellow line delivering vacuum into the ventilation system all right, but the actuators seem to be located primarily on the right side of the car. Remove the panel over the passenger's legs and, I think, you will see several vacuum actuators quite clearly. The manual specifically recommends carefully checking the hoses and connections as well as the actuators. Hopefully, this will help get you started in your checking. If you don't find the problem and want copies of the manual pages (which go into considerable detail), let me know and I will try to send them to you.

Good luck. Hope this proves helpful.

__________________
Ted
1979 240D
165,000 miles