Sounds like you definately have water getting past your door membrane for the door card to be getting mould on it, the water could be just beneath the sound deadening pads under your carpet where you can't see/feel. it only takes 2 minutes to get under the front carpets to have a look, just remove the metal trim holding the carpet down by the door and lift the carpet, you will find that as your car warms up the moisture in/under your carpets will start to evaporate, thus steam your car up. Failing that you could have a microleak on your heater matrix which further adds to a damp environment inside. I rather fancy leaking door membranes letting water sneak under your carpet but just not enough to show through. When you switch your car off at night especially on these cold nights all the moisture in the air condenses and migrates into your seats and carpets just like onto your windows giving the damp feeling you are getting. Take a look under your carpets first point of call and check your membranes then battery tray. They also like to leak through the tail gate electric cable boots which water then gets under the boot carpet and under the back bench, these turn into a giant sponge and give off steam as the car warms up.