Might check and see if the switch, that I can't remember where or what is but is probly on the hvac unit, that activates the AC isn't malfunctioning.
As stock, it'll trigger the aux fan when the heater/ac fan is activated and the lever is in AC or defrost.
I had a failure once where when the aux fan kicked on for temp control, it activated the AC compressor, the diode in the system had failed.
I had a failure once where if the aux fan was on for temp control, and the hvac fan was on, the aux fan would stay on after the temp switch
kicked off, untill one turned the hvac fan control off. I don't remember how, or if, I fixed that one. Don't think I did..