ac/heater fan switch only works on high

there is noway to fix that minor issue. just deal with it or go to a junk yard and get a replacement or go buy a new one.

The resistor has gone bad, replace it with a new one and also get a pigtail to replace the plug that plugs into the resistor, they tend to go bad as well. The resistor is under the dash on the pas side next to the motor and if I remember correctly you will need a 8mm socket on the 2 screws that holds it.