I think the problem with electric car heaters is that they just can't supply the same amount of heat that a conventional one plumbed in to the engine cooling circuit can. They are hundreds of watts, rather than thousands. To supply the same sort of power electrically would overload the alternator. Think of a one bar domestic electric fire, 1KW. That would be 71 amps at 14V. Too much for most alternators, especially since they have to run the engine electrics, lights, screen demist, etc etc at the same time. The amount of waste heat from the engine is many KW. The issue with that is more getting rid of it, as the many cooling problem posts on this forum show! Diverting some of it to heat the car interior is simple.
Jenny
(Corrected amps for watts. Doh!)