89 626 temp guage not working

my sister has a 89 626 with a non functioning temp guage. the engine seems to come up to normal operating temperature and the fan kicks in when letting it idle in the driveway and no sign of overheating. the wiring harness has three connectors going to the thermostat housing, one round, one flat and one unused connector. when i disconnect the flat connector before the engine warms up the fan comes on so i'm assuming that the temp sender is ok. checked all the fuses too, any ideas?

look for wiring around the unused sensor the round one is for the efi i think but if you ground the wire the gauge should go to Hot .
the one you disconected and the fan went was the fan sensor it opens when hot to operate fan