The fan switch should have a setpoint higher than the thermostat. So it won't come on until the temperature climbs above normal. (But not all the way up...)
Unplug the fan switch, jumper it's wire with a paperclip. Key on, this should make the fan(s) come on. Check this first, if it works, then the rest of the system (fuses, relays, etc) are OK.
Call around, I can't believe that fan switch is all that expensive ($500?? wtf..) If you really don't wanna buy a new fan switch, take it out & make up some jumper wires to a multi-meter. Stick the switch in a pot of water with a thermometer. Heat it up & see what temperature the switch closes at.