Andy, the sensor should supply a ground.
No sensor, no ground.
That means the controller thinks the car never gets up to temperature and the fan never comes on. Engine go *poof*

A fail safe would mean no resistance to ground (missing sensor) turns on the fan.

The other plan would be to supply a ground via manual switch to trigger the fan.

See the problem? One says no ground turns on the fan. The other says no ground, no fan. It would require logic to be added that determines what to do when either switch goes to ground or remains open and which to ignore.

A dipswitch would tell the controller which program to follow.

We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon