On the theme of cheap stuff for the car... I've been looking at my Target fleece sleeping bag rolled up on a shelf in the garage, and thinking that might be better in the trunk.
Unzipped, it could be used as an emergency blanket. Zipped, it would work as a minimalist thermal barrier.
I also carry a 7 x 9 tarp and a big (queen-sized) velour blanket. With a hank of paracord, I think I have the shelter stuff pretty well covered. (I'm in Florida, so the cold isn't as big an issue as in other places.)
Bear
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No fire, no steel.