Orientation and surroundings of the vehicle should be considered. Sunlight entering a cleared window might warm up the interior, and snow left on top of the car may help insulate.

Snowholes aren't warm, as they usually stabilize right around the freezing point, but they may well be warmer than the vehicle.

You might want to rotate between a sun-warmed vehicle during the day, and the snowhole at night.

It would certainly be interesting to see some research on this very subject. Any volunteers? <img src="/images/graemlins/grin.gif" alt="" />

Sue