MB 1990 300E 24 - more help please!

Hi - just got this car (to replace my old 190) - only thing wrong with it so far is with misting on the inside of the windscreen. What it boils down to
is that the fan / heater / air conditioning works fine (according to the manual), but it isn't blowing onto the windscreen - it is blowing through the side and centre vents and the foot well. All I'm getting from the windscreen vents is very poor air circulation. The air distribution switch works fine in switching the air to the vents and the foot well, but it won't switch it to the windscreen! Does anyone have any ideas what is wrong (and how to fix it)?
thanks Andy
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