Are you running with the cabin air intake set to recirculate, if so this prevents a flow of air through the car and builds up the water vapour in the car which will condense on any cold surface ?
Set the air intake to outside air and run the fan speed at no lower than position 2 and never turn it off. Run the heater at maximum for as long as you can stand it and if you have air con turn it on to dry the inside of the car out, don't believe the green comments on fuel consumption.
Vic.
I have the air intake on outside and ALWAYS on position 2-3 , + all the time aircon on, this is the only way I can get back my windshield

but it never seems to cure it anyway,always after few hours the window is fogged up with loads of water, UK climate doesn't really helps this anyway,in summer I never get this problem.
Filled up few socks now with cat litter and put it in car,let's see if this can help a bit.
btw the aircon DOES uses Lots of fuel , probably ~15-20% more than without it.