its probably the switch, youre gonna have to use a multimeter and a wiring diagram to see if there is voltage coming out of the switch when its in low and medium.
Blower motor resistor. They fail to HIGH...so when it goes, your blower will only work on HIGH.
The resistor is bolted to the evap case near the blower motor. You should be able to access it from behind the glove box....drop it down and you should see the resistor held into the case by two bolts.
The resistor pack receives full current from the circuit, and like a stepped dining room light rheostat, the resistors are what knock current out of the circuit. The diminished current spins the motor less quickly, giving you speeds.
This might make it more logical why it works on high but not other speeds.