[dcb]

(another approach)
lets say you have a 2 liter engine, cruising at 2000 rpm. Lets also say the throttle is not very open for a %30 Volumetric efficiency. And you want to raise the air temperature 50C degrees.

So your engine is trying to move 1000 liters per minute, but the throttle is limiting it to 300 liters per minute.

Lets say a litre of air is 1.2g/L, so you are trying to raise 360 grams by 50C, every minute, which requires 18000 calories (not kilocalories).

Lets say (after some more googling) that a calorie = 3.08596003 foot pounds (yay), so we need 55547 foot pounds per minute worth, or 926 foot pounds per second.

Well, a horsepower is 550 foot pounds per second, so this heater requires 1.7 hp for input, so if your alternator is (swag) %85 efficient, you will be taking about 2 HP from the crank to run this heater.

A general rule of thumb is that your (not huge) car only needs about 10-15 hp to maintain speed on the hiway, so 2hp is a very large chunk of power just to run a heater (when exhaust heat is free). You will need to draw almost 100 amps for this example.