The nice thing about the propane furnace above is that you can get 60 hours off of a 20lb tank. I'd need a how-many-amphour-battery to get just an hour of heat?
For a 1500 watt heater you'd need too many AHs. For two 200 watt ceramic heaters, I think the draw was about 20AH for one hour. By placing them at floor level I was able to get tolerable heat in the driver's footwell at cold temps. If you want to keep passengers warm as well, though, you might need the propane heater. Just remember that regardless of some advice here, you really should vent it out of the car.