Thanks again for the help. I finally got hold of a wiring diagram for the car. as you mentioned, two thermostatic switches are integrated to trigger the fan. One of them is supposed to turn on the fan at 100 degrees C and is located on top of the thermostat housing. The other one is labeled thermostatic switch for dehydrator (option ac). As suggested, I grounded the wire at the switch but the fan doesn’t turn on. If I ground the wire at the relay plug, the fan turns on, no problem. Concluding, I believe I have a wiring issue between the switches and the relay plug. (Anyone know where the thermostatic switch for dehydrator (option ac) is located and what it looks like, didn’t find it yet).
I am also tempted to just add a manual trigger switch hidden somewhere to turn the fan on manually.