I've covered this before. You are very unlikely to freeze to death, even in very cold temps, if you just stay in the car. I no longer travel in extreme cold, but I still have a couple quilts that are always in the car. Even with no heat, a modern car will minimize drafts from cracks in the cabin. Wrap yourself in the blanket and if you have a traveling companion, bunch up with them, inside the blanket and you will survive.