If only one position does not work, odds are that the switch is shot. If you pull the switch, you should be able to see.
Another way to verify if the switch is good is to probe the different positions on the switch with a voltmeter. IIRC, the way it is wired is that the switch will send voltage to each position, one at a time. While it is possible for the fan resistor to have a single tap not work, it is very rare.