I live in Iowa, and every Ford I've owned - and that's quite a few since I only drive Fords - has fogged up / iced up the inside of the windshield. I'll clear the outside and the inside will be clear until a short time later when it ices up or fogs over, often after I start down the road. Dangerous and aggravating for sure. My STs have had this problem less than any of the other Fords I've owned. The '74, '77, '79 Rancheros, and the '78 F100 were the worst. I don't know of a solution, although recently, I read that a mix of vinegar and water sprayed on the outside of the glass the night before will prevent frost from forming on the outside. Might work on the inside too. It was going to be around zero at the time I read that, so I didn't try it, figuring the solution would freeze on the glass at that temp. The mix was 75% and 25%, but I don't remember which was water and which was vinegar. Rain-X on the inside is supposed to work too, I think.