body warmth and skin on skin contact is best for warming..
you'll come back a changed man
dont sleep in your car with propane or gas heaters and asphyxiate yourself. find a way to stay safe while staying warm, air matresses dont hold as much heat as foam while sleeping, booze always helps too.
lots of people do this in winter, paticularly the crusty die-hards who are awaiting first turns after big dumps so good luck. car camping just takes some experience/equipment so don't expect to be great at it your first go.