I was in the Air Force in North Dakota for 4 years. We were trained to carry a survival bag for each person in the car. Enough blankets and clothes to survive overnight in way below zero temperatures. The recommendations back then (early 70s) were to watch your fuel and run the engine with the heater on if possible. If it was snowing or there was blowing snow, have someone get out and check the exhaust to make sure it wasn't blocked. Crack a window on the leeward side of the car for ventilation.

We were also told to carry candles to light in the car (with a cracked window) if we couldn't run the engine. I never had to use any of this, so I don't know how well candles worked as a source of heat.

The issue I have with being stranded in the winter is that you won't know how long it will be till you're rescued, so providing heat in the cabin of your vehicle is not a good solution - you'll run out of gas, propane, candles if you aren't rescued promptly. My idea is that I should have sufficient gear in the car to keep us all warm enough to survive our coldest temps without further heating. I'd say body heat is the best, most renewable, and longest lasting source.

We also carried food with us.

I have to say the gear the AF issued to its members was excellent. I was out in very high winds at way below zero at nights and while it wasn't pleasant no one was cold in our winter outfits, not even toes and fingers. Now I live in the San Francisco Bay Area and it hasn't been below freezing since we moved here in '94.