I like this idea, though it may not work all that well. A typical alternator puts out between 50 and 100 amps. At 14v, that's about 700 to 1400 watts. However, some of that power is used to run the engine. I doubt you are going to have more than 1000 watts to power the heater. That doesn't sound too bad, but an average hair dryer has more watts than that. So it won't really heat up the car all that fast. Still, it would help at least some. I would like to have it in my car.

When I was in high school (early 70's)a friend of mine had an old pickup truck, 1947 or 1949 I think, that had an electric heater.