A sleeping bag is essential. A decent bag occupies about one cubic foot of space, weighing perhaps three pounds, so I don't understand how one could not fit in any vehicle. I have always carried one, beginning with a 1962 VW Beetle. Two heet sheets will definitely not be adequate, although they will probably help you avoid amputations due to frostbite.

I would include a flameless cooker or MRE heaters. Might as well have a bit of luxury. For that matter, put some really nice items in your food stash - really, really nice chocolate bars, for instance.