It sounds like your air conditioning is not turning on when it is supposed to, as the defrost mode is supposed to turn the heat on full and the A/C on full, and direct the hot, dry air to the windshield. When it is working this is very effective at drying off the windows.
On my 350SD if the car got really wet in a rain storm the A/C would sometimes shut off. I had to pull over and shut the car off, then restart and the system would work ok. I never figured out if that was normal or not, and it did not happen all the time, only when it really caused some anxiety. In a heavy downpour the last thing you want is to have to pull over, shut off the engine and restart the car so you can see the road.
I think your car may have the same issue, and it may come from the liquid level in the A/C air cooler box from condensation rising too fast, possibly due to a sluggish drain. I don't have the car anymore and have not experienced the problem in the manually controlled system on my 190E, and have yet to experience those conditions on my 1988 300E or my 1998 E300D TurboDiesel.
I hope this helps you. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)