All good ideals altho I would think the taillight bulb would probably kill a good car battery pretty quick, matter of fact any one of these ideals would kill a car battery pretty quick without some sort of chargeing system. Probably the most efficient method would involve some sort of Positive Temperature Control. Lookup PTC heaters. One could use the nichrome wire sized to the correct length, with a power regulator to construct a incubator that would hold a constant temp at a preset voltage. The voltage regualtor would be needed to to prevent spikes in temps due to the charging of the battery system. More amps during chargeing would mean more heat. My guess is a incubator that had a continuously wound nichrome resistance wire throughout the cabinet, (sort of like and electric banket), wouldnt need a fan to circulate heat and would reduce the power consumption required by a fan heater combination. The cabinet would also need to be well insulated, yet still provide for fresh air exchange, Maybe a convection type of air exchange where the cool air is drawn thru the bottom of the cabinet and the hot air vented thru the top. Would probably also need some sort of large surface area water container for humidity control. You wouldnt have the benefit of a fan blowing across the water to raise humidity.. At anyrate, I think one could be built ( and it looks like a few have been built), and ran off some sort of power inverter along with a battery backup for power outages.