The reason the blower works on high speed is that there is a relay built into the system to bypass the resistor when the fan switch is in the "high" position. An open circuit in the resistor stack, or a dirty switch connection or burned/corroded wiring will disable the slower speeds.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!