Thanks a lot for the reply! Very helpful. I would like to restore the stock system to operational, then maybe add an additional thermal switch that comes on at a high temp as a backup. My friend uses his car for his job & he drives it pretty hard, so bullet proof would be nice.
I will check the fuse & change the head temp sensor. Is there anything else that can keep the fan from working? Like air bubbles?
For your thermal switch back-up, that would work fine. Since it hopefully won't ever be needed, you can probably just tie it into the high speed fan relay input (Pin 17. It gets grounded to turn high speed on). Or also use your own relay and tap into the load-side of the high speed fan circuit.
Air bubbles can affect sensor and sender readings, which would in turn affect the fan. If the system hasnt been opened of late however, chances of air being a factor are slim. Most of us have aftermarket gauges so we know what's going on. Even if temporarily installed and removed, or mounted in the engine bay (if the owner doesnt like aftermarket gauges inside his car), this helps a ton. They even have gauges in radiator caps these days. The stock gauge is just kinda tough to interpolate (esp if one doesnt have a cross reference like the ability to read Pids or datalog, etc).
Couple more random things you can try: if you run self-diagnostics, the first thing the car does is cycle each fan speed. This is a quick tell to see if the circuit is intact.
If you disconnect the ECT electrical connector and start the car, the low speed setting should come on.
If you ground CCRM Pin 17, the fan should come on high.
If you apply fused 12 volts to CCRM Pin 14, low speed should come on.
MAX AC should make high come on.
Checking any or all of that can be telling in narrowing things down.
Good luck.