Screen steams up in damp conditions or raining.

I would check your coolant level to be sure you don't have a leaking heater matrix, if this is ok check that the cabin filter is'nt damp

coolant level ok. Had the pollen filter out and it's dry & undamaged. Where does the rain water escape to. There is a vent each side just below the wipers, it seems to go under the dash. I was wondering if something was missing.