OK... your aux fan should come on at low speed when your car reaches 100 degree. The ECT sensor should feed 310 Ohm to the harness and therefore, the car knows its 100 degree thus turning on the fan.
Here is the test, unplug the harness from the ECT sensor. Than get a 310 Ohm resistor, or something lower to be sure like 200 Ohm, and cross the connector. This should turn the Fans on. If a 310 Ohm won't turn it on, a 200 Ohm must turn it on as it should register something way beyond 100 degree on the car computer.
Test your ECT sensor too. Just get a Multi-meter and check for resistance across the 2 pole on the ECT sensor. At 100 degree it should register 310 Ohm.
Once you confirm the fans as well as the sensor are working, the next thing to test is the different stages. The car have either 2 stage or 3 stage fans. each of these stages kicks in depending on temp thus Ohm from ECT sensor. If I am correct, its 100 degree for 1st stage and 2nd stage at higher degrees. Again I don't have detailed spec with me but I am sure other members know. Find out what Ohm will trigger each stage and start testing by crossing with the approporate resistor.