Run a search for climate control, there are at least four threads dealing with this same issue. I'm having the same exact problem with my 94 C280, and it is probably a vacuum issue, according to many of the posts. Unfortunately, it's snowing pretty heavy here and I am not going to check it out today (I'll be driving my 4WD truck). My problem started last week after going into a car wash that stuck while I was under the blow dryer. Since the air inlet is at the bottom of the windshield, I think that the constant high velocity air flow screwed something up. The problem is extremely annoying, and I'm sure a dealer would attempt to talk you into replacing the whole climate control system. Good luck!