I seem to recall a post quite a while back that suggested that the low speed had something to do with the resistor. Is there a way to test the switch to figure out why the low speed isn't working.

Educated guess says it is the resistor as the high speed is 12v without going thru the resistor. Other speeds go thru the resistor so the voltage is "stepped down". When the resistor goes it's usually an open circuit thru it. I think to test it you need to remove it and for what they cost you could replace it at that point. Someone else may have a better suggestion though.