The switches do go bad, but it could be a bad resistor (if only low speed isn't working), a bad wire, a mis-wired switch, rusty connections or a bad ground.
First thing to do I suppose is check the fuses. You can just check the heater fuse of course, but I would pull them all and look for rusty contacts. Clean them and put the fuses back in. Replace any that are rusty or suspect.
Next would be to check for power at the main power wire to the motor when you turn the switch on. You have a volt-meter or test light?
Remember to turn the key to ON to do this too, as nothing in the heater is getting power without the key.
If no power to the wire, then you need to make sure power is going to the switch. You can pull the connector off the switch, turn the key ON again, and check for full battery voltage. If none there, keep moving down the line. If you do have power at the switch though, then your switch or the wire to the motor (after the resistor) are bad.
Paul
