Running your vehicle's engine to operate the heater is tempting, but futile in the long run. As soon as you turn off the engine, the vehicle's interior will get cold almost immediately due to the lack of insulation. Moreover, there is a real danger of death by carbon monoxide poisoning from breathing in the emissions generated by the engine exhaust. Besides, if you're going to be stuck for very long, you're going to run out of gas anyway, so try to figure out other ways to stay warm (lighting a small candle can produce an amazing amount of heat). Just remember to crack the window open a little, so you have fresh air to breathe.