For sleeping in my Jeep Cherokee when cold, my son came up with this idea and it really works! You know the cheap clear plastic bubble rap? Cut it to the size of your windows you want to cover at home before you go out. Take a wet rag of some type and wipe the flat back side of the bubble wrap. Apply the flat smooth side to the windows! Stays in place well, and does not seem to have the condensation problem like the foil sun shield type. Does a better job all around I think than using the foil type. Nice to have some light coming in the windows depending on your camping, and staying warmer too.
Off the topic a bit. With that said, as a younger Man I did a lot of mountain climbing. Still have Two mountain class of tents designed for use in cold, nasty, high winds type weather. Never did use anything other than some cooking to warm the tent. Because of your breath alone in the sub freezing temps, I would use a tent frost liner. The frost liner would keep the tent much dryer, having your breath freez to to the liner and not have it almost snowing in the tent from condensation. It made a big difference! Also, make sure you have a good sleeping pad that keeps you up off the ground or cold surface, does not absorb wetness, and will not compress much but insulate your back side when sleeping. Before I go to bed, I change into dry long underwear, saved just for sleeping. I make sure I have dry long Soft wool long underwear on with dry wool socks. Cotton is not your friend in cold weather. The polypropylene long underwear works well too. Hard to find the good high quality wool long underwear anymore. Winter snow camping is still some of my favorite, but like most things having the right gear makes all the difference in staying warm and having a good time snow camping! Benz.