How much horsepower does the typical automobile heater blower draw via load on the alternator?

Answer 1

1/500th or less. You can check this out by looking at an auto supply catalog, finding out how much current is drawn by the fan under loaded conditions, determine the current draw (presumably at 12 volts), and find the wattage.

This can then be converted into horsepower by multiplying watts X 0.00134.