Having an intact car for shelter is better than not having it. I've used my car for overnights on a few occassions and it does get cold inside, but not as cold as outside.

I don't understand this tearing up the seats for insulation thing. If you're thinking and planning, supplementing the car with sleeping bags or blankets seems like a better use of resources than dismembering your car. After the storm you may be able to drive out if you still have a car to drive. I've added a cheap fleece bag to my kit which along with the cheap "Italian Officers" wool blanket I got through one of the on-line surplus dealers I should be able to stay quite warm. Another body/person inside the cab would just add more shared bodily warmth.