Heater resistor?
... what exactly does the heater resistor do?
The heater resistor is what gives you your fan speeds.
No resistor, straight from the switch is High.
Then two resistors (one with a tap, actually, if I'm not mistaken), each fed by a different wire from the switch for Med and Low.
More resistance = less speed.
The resistors get hot, as they are fairly low resistance and the motor draws a decent amount of current, so they are placed in the air stream of the blower, to keep cool.
However, over many years and miles, usually they're not kept quite cool enough, and something melts, and *POOF!* no more Med and Low.
They used to be a dime a dozen at every auto parts store, not sure now, but any heater resistor from any car ever made can probably be adapted, and ones with resistance close to the original should work identically (otherwise your Med and Low could run a bit faster or slower than you want).
As stated above, if you don't have High, then the problem isn't your resistor, so check grounds - just jumper a metal part of the motor to a clean metal part of the car and see if it works.
– Eric