Will this work?
http://www.justanswer.com/ford/1grjp-cr ... f-fan.html
I haven't tried it, but several references say it works:
To turn on the fan with a switch:
Add a switch to the dark blue wire in the fan connector at the top of the fan shroud. This connector is a five wire connector and the dark blue wire receives a GROUND connection from the PCM when it decides that the fan needs to be turned on. After turning on the switch to send GROUND to the dark blue wire, the viscous fan clutch heats up (it will take 20-50 seconds or so to fully heat up) and the fan will engage and run with a lot of sound!. After turning off the switch, the viscous fan clutch will still be hot, so it takes a minute or so for the fan to slow back down. Again, the dark blue wire is for GROUND only, not 12 volts! Also, the heater in the clutch draws about 1.25 amps or so. You can use a 16 gauge wire and a small toggle switch. Just make sure you do not cut the blue wire such that the PCM can't ground it when it wants to