123 humid interior when in "EC" mode

[The Warden]

Hello! This is actually my first venture into this forum, having mainly hidden down in Diesel Discussion. However, this is a question that, I suppose, would relate to any Benz, and no one down there seemed to have a definite solution, so I figured I'd post it on here.

As my signature says, I have an '85 300D that was originally a Texas car (the car lived there until about 7 months ago). What's happening is that, whenever I have the ACC set on "EC" (standard heater mode; a/c compressor not turning), the interior gets very humid (much more humid than the air surrounding the car). The windows fog up within about 15 seconds to the point where I can't see and everything gets sticky, regardless of where the temp wheel is set or what the outside temperature is. Once I switch back to the regular mode (with the a/c compressor turning), the humidity goes away in very short order. I know that part of the process of air conditioning is the removal of moisture, but my concern is where the moisture's coming from in the first place. There's no sitting water in the car (finally got that fixed ) and the a/c drain hose has been replaced and there didn't appear to be any blockages in the drain leading up to the hose.

It's at a point where I can't use the "EC" mode at all; even if I turn it on without having had the a/c on before, the car gets humid. In addition to replacing the drain hose, I've also put a repair kit in the monovalve (solving a different problem). Any suggestions would be greatly appreciated. Thanks in advance!