Perform a functional test with an automobile scanner is the easiest way.
The 1996 Mercury cooling fan relay switch is on the back of the cooling fan motor. The relay switch should be labeled as such.
On a 1998 Mercury Tracer, the cooling fan relays are located in an engine compartment box. The compartment box should be labeled 'Cooling Fan 40 amp.'
under the dash behind glove box
The temperature sensor goes right on top of where the water hose from the radiator plugs into the engine. it is right next to the spark plugs wires.
I had to bypass mine and set up an internal flip "on/off" switch on the dash.
The thermal switch for other Mystics is located on the thermostat housing.
That would be your LOW engine coolant light You can view the 1999 Mercury Mystique owners manual online at : www . motorcraft service . com ( no spaces ) Click on Owner Guides
It's on the fuse panel under the right side of the dash.
location of engine cooling fan sensor
If the AC is on, this is normal.
Evidently the cooling fan is not working.
check your relay for the cooling fan and fuelpump