The common failure part in that system is the power control module. Since the fan comes on when the thermistor is disconnected, that indicates the thermistor being bad.
the fan will come on at 2.5k ohms (260 degrees)
the light will come on at 1.6k ohms (290 degrees)
when the thermistor is disconnected does the hot light come on also?
the box is the power module. they had some recalls on the power module boxes due to burning up. was yours replaced? if you are not sure if you unit is part of the recall with the vin I can confirm it for you if you would like me to.
was their any thing else you kneeded to know?
you will need to check the resistance of the thermistor with a thermometer hot water and a ohm meter. or it may be easer to replace the thermistor if you think that is a bad part. the readings should be.
100k ohms (77 degrees)
3.5k ohms (240 degrees)
2.5k ohms (260 degrees)
1.6k ohms (290 degrees)