I have been following your dash fan problem because I also have a speed problem. I tried Evans but after contacting them, it was determined they were not the supplier. It turned out to be Frigette.
They both use the same Ford control panels so it is not obvious who the supplier is. My problem is the fan will only run at Hi speed. I found the resistor on the front of the heater/air condensor plenum in the engine compartment but is tested OK. I just down loaded the wireing diagram which Frigette sent me and I see a Relay in the circuit which I had no knowledge of. From the testing I have done before getting the diagram I believe the relay will be the problem. The Evans system does not include a relay, at least in the wiring diagram. Now all I have to do is locate the relay.
If you have not solved your problem, I hope the above link will help.