Ages and ages ago i had an escort that was the same. Turned out that the battery was being overcharged by a dodgy alternator, leading to acid vapour eating away through the battery area compartment making holes and allowing rain to come straight into the car.
if the car si water logged, demisting will be almost impossible, makes sure you have the air con on and heat as hot as u can bear to help clear it.
Trev
May 2007