On the driver's side behind the headlight you will find the air conditioner drier - a black round object. On the top of it you will find a sight glass and a sensor that is green in color screwed into the top beside the sight glass. The two wires to this sensor should cause the fan to come on if they are shorted together as the green sensor activates when the a/c pressure goes high enough to engage it. Mine did not cause the fan to come on when they were shorted together. Now - miraculously - they do when shorted together. The temp sensor is on the top of the engine at the end of the upper rad hose. There are two sensors here - one with one lead (temp guage) and one with two leads - Aux fan controller. The system is designed so that if the sensor goes open - the fan starts on high. So to test it - simply pull the connector off the sensor and the aux fan should start right up - when the engine is at operating temperture. If it does not - change the sensor - This one is easy - only slight antifreeze loss. The green one on the drier screws right into the a/c system and the system has to be evacuated and recharged to change that sensor.
Hope this helps you. I am leaving for a funeral in Michigan this week for my sister who died of ALS - so I am hoping the repair holds - if not - I will patiently check things out when I get back and find it once and for all. By the way - when I had pulled my temp sensor on the rad hose - prior to the fuse and relay thing - my fan did not come on high speed - it does now.
\Morris