If the heater fan goes out a little at a time, then finally quits, is that the failure of the motor or a switch or something else?

 

If they are up to spec, the thermo-switches that control those fans are very precise in reading temperature. It is not unusual for them to cut on and off quite a bit. Even the switch that makes the fan run to disperse accumulated heat after the engine is off might cut in and out a few times before it is happy with the temperature it is sensing.

If you have AC, do both fans run while you have the AC on?

 

The fan does not function for either AC or the heater. Actually, the AC went out a few years back.
The entire AC unit was bad, and I didn't want to invest in a new unit. The heater fan went out
just a few weeks ago, but was working very weakly before that.

 

The fan worked with both A/C and Heater, then little by little it grew weaker, and finally quit.
Is there a definitive answer for the origin of the failure?