I had the same symptoms on my wife's '91 190e. I just fixed it a month ago.
Test the heatermonovalve first then start testing the vacuum elements under the dash. In my case, the monovalve was good but I had two bad vacuum elements under the dash: the defroster element and freshair/recirc. element.
I took the dash off and replaced the two elements and also the heater potentiometer while I was there(they frequently go bad).
Everything works now. Let me know if you have any ?'s. I wish I had taken pics while I did it...