Electric window defrosters are really low wattage - because its just trying to raise the temperature of the surface of the glass by a few degrees. Generally IIRC no more than 25-30a - which is a fair bit on a car with a 100a alternator - but less than for example the blower for the air conditioner.

I never thought I'd see the return of the gasoline heater. I wonder if its offered in the states.