If you drive with the heater on, then warm air is able to hold more moisture, and there will be lots of it from your breath, especially if on recirculation.
Dewpoint will be a lot higher than normal and condensation will occur sooner as the air cools down. On a really cold day the air is usually quite dry, so when you park up, open windows/doors to purge out the warm moist air before you depart.
Longwinded theory I know, but give it a try, it might improve things.