Just checked the resistor. I’m getting voltage across the resistor, that tells me the resistor is intact and the switch has failed. I was under the impression that the switches never failed, but seems pretty clear. Let me know if I’m lost in the woods.

 

What kind of probe is this?

 

Ancel. It’s a knock off power probe. I use it to test circuits. Way more convenient than an ohmmeter.

 

This blog and the referenced wiring diagrams in it helped me with my 66 that has a 3 speed heater system.

Mustang heater blower motor 1965 and 1966

Here is how the heater blower motor works on the 65/66 Mustangs. If you click on the diagrams, they will pop out so that...

thecareandfeedingofponies.blogspot.com

 

Great read! I’ll clean up the switch terminalAnd check the clearance to the dash. Crazy the damn thing is grounded by touching the back of the sensor.

 

When I added in-dash factory style AC to my non-AC 1967 it confused the heck out of me to find that the power and ground were reversed to the heater motor. I may be mistaken, but as I recall the resistor was on the ground side for no AC and positive side with AC. It helped that the AC wiring harness was a completely separate add-on.

 

Sorry for the late response guys. Started drinking the Kraken and eating pizza/wings. Next thing ya know....

 

Ok, thanks guys. We don't know why Henry did this, but it is shown below schematically. It starts at the ignition switch, goes through a fuse, to the motor, then either through or around the resistors to the switch where it goes to chassis:

 

I’ve got voltage back to the switch. Easy fix. Probe made this a fairly simple process over the multimeter. Thanks for the input guys. I’ve seen ground activated systems before, it just takes a couple head scratches to sink in.