Since winter tires are, and should be, the obvious #1 answer, and pretty much every car has heated seats these days, I’m going to go with ‘heated steering wheel’. I thought this was a silly thing until I had a service loaner Audi A4 with such a wheel…it was a revelation. Instead of terrible bulky gloves, I could wear my regular thing driving gloves and still have warm hands! And it’s a safety feature, too…again, bulky gloves not as good for grip, but also cold leather tends to be slippery, so warming it up helps it stay grippy.
But for things that I ADD to the car for the winter…ever since I started driving, I’ve always had a fold-up shovel. My first was my dad’s old Army ‘entrenching device’ that he insisted we each keep in the car, but I have always had once since. All it takes is needing it once…and I did. It also was handy as a potential weapon when I broke down in the woods and there were black bear around (whether or not it would have been useful is besides the point, it made me FEEL better to have a ‘weapon’).
I also always have a blanket…but that’s a year-round thing, so I didn’t really count it (handy for an impromptu beach visit, picnic, etc.)…but it is also a good answer. I also have a couple of packs of hand-warmers in my toolkit, so that’s another idea (they have battery ones that also charge your phone, so not a bad idea there). On my bike, I always have spare gloves, but those would be useful in a car, too.
I cannot ethically endorse the use of a folding military shovel to fight off bears. I can, however, postulate that it’s probably more effective than trying to fist fight one. Your bare knuckles will be ineffective against their bear knuckles.