There is also some stuff called rainX. Halfords do there own version I believe. Anyway it's main purpose is to coat the screen and make it so slick that water droplets cannot form or stick on the windscreen. Great stuff actually cos I remember going down the motorway when it was chucking it down, and my screen was so amazingly clear.
That said, a by product of using it is that bird poop is easier to just wipe off, and in cold weather, ice does not form on the screen. Well it sort of forms, but does not really stick, and can just be lightly brushed off.
As for inside the car, as I believe was said, it could be seals etc, or maybe a window slightly open. When practical, try running the car while stationary with a window slightly open and the heating full on for a while - this will help "dry" and moisture inside the car and the open window will give that a means to escapse rather than keeping it in the car.