Jumper cables, because batteries are more likely to die in the cold.
Tow rope so somebody can pull you out, or you can pull others out - I've done it many times, and others have done for me. Get a J hook or you probably can hook anywhere on a modern car.
drinking water
Small shovel - no way you're digging out without it.
Enough extra clothes, especially warm gloves and hat and footwear, to wear while digging out.
Personally I carry a -20 F rated sleeping bag not a thin blanket, because I ski in remote back country, so if I get stuck I need to be able to survive in 0 F or below for multiple days.