[vettestr]

Many fan circuits are wired hot and the ground is added/removed with the switch and temp sensor. If yours is wired this way here is whats happening. Key supplies B+ and then the temp sensor reaches trip point so enables the ground and fan turns on. If you want to turn fans on before it has reached the set point you turn the toggle switch and it supplies the ground source so fans come on.

I think you have a switched ground system and something or somebody has defeated the temp sensor, so it is grounded always and the fans always run. Get a volt meter for as little as ten bucks so you can check polarity and see if voltages are present to be able to check things out.

With the tester/light you now have you can not test a negative switch but doing test below will not hurt tester of the circuit, it simply will just show red all the time. You could use a known B+ source and sw. but much easier to walk ya through a VOM.

I think you have a bad sensor or if adjustable it is tweaked as low as possible and always on. Fix the sensor and you will be good to go.... Of course this is only cause I stayed in a Holiday Inn last PM