that'll be cos the car is damp inside
i recently had someone view a fiesta i had and they didnt buy it cos the windows wouldnt clear but there were 3 of us in the car all with wet coats and it was pissing down outside

find the leak (s), check the bulkhead seals battery tray and the floors as in this weather alot of spray hits the floor and comes in thru holes and seals that are going a bit and water coming tho under the dash cant be good