I was looking at the electrical schematic today and noticed the aux fan is controlled by two things, 1) the water temp switch located on your thermostat housing and 2) an ACswitch, location unknown ATT.
the water temp switch is valued at 100deg C or 212 deg F. If you stick it in a pot of boiling water with meter hooked up you could verify it's operation. Also you can hit the fan with 12v from a battery charger to test it, that's how I tested mine. AC switch ?