What is causing a condensation problem in my 2001 SEAT Leon?

I am trying to sell a SEAT Leon 1.6S Petrol 2001Y with 68,000 miles on the clock. However, it seems to have a condensation problem, especially in the cold weather - the windscreen is running with water inside and it smells damp. Could the air vents be blocked up somewhere? Will this prevent me getting a reasonable price for the car? What is the car likely to be worth if sold privately?

Asked on 15 February 2011 by SW, Chippenham

Answered by Honest John
The waterproof cover over the pollen filter in the bulkhead vent well is either cracked or has not been properly replaced after the pollen filter was replaced (it's a knuckle grazing nightmare to fit and appalling design). That is allowing water into the car which will first saturate the passenger footwell carpet. Fixed, as a private sale, the car is worth £1,100 - £1,400.
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