I've had some fun with this lately too!
My car has twin fans. Only one was coming on. In my case, unplugging and re plugging the cables to the resistors sorted it.
The fans are controlled by relay switched earths. The fans have constant positive power connection. The relays are activated by the ECU based on the temperature signal from the thermostat temperature sensor.
Disconnecting the thermostat temperature sensor should trigger full speed on both fans.
The fans have two speeds. Each controlled by its own relay. The first speed is controlled by the two resistors as a fail safe, each resistor controls one fan.
If you have access to fiat ECU scan or multi ECU scan you can test the activation of the fans that way. Otherwise a multimeter may help.
I'd say that if the fans aren't coming on at all, yet the temp gauge in the car is working, the wiring to the fans, or the fuses are the most likely issue.
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