What could be causing my Nissan Note's windscreen to steam up in wet weather?

I purchased a Nissan Note two and a half years ago following your recommendation. I am extremely pleased with it as a versatile, comfortable and economic car. However whenever there is a sniff of rain in the air, the windscreen steams up rapidly. I questioned the dealer whose recommendation was to leave the air con on. (In fact, whenever positioned to defrost, the air con comes on automatically.) I feel there may be trapped water in the ductwork, but the dealer discounted this. It seems crazy to have the air con on in winter to defog, when for comfort one requires the heating. Have you heard of this before?

Asked on 5 March 2011 by CD, Tavistock

Answered by Honest John
A/c does dry the air inside the car. But it's possible that either the pollen filter is blocked with dust, or it has become sodden because the drains from the bulkhead vent well have become blocked leaving rainwater nowhere to go except via the pollen filter into the car.
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