thats a good one i had a problem with mine after a complete engine rebuild and car restore,to put it simply the magnetic clutch on the pump wouldnt engage,turned out to be the throttle position sensor out of adjustment,i shit you not
the main fan/aircon selector switch is a complex unit inside and it may have a short internally which is causing you strange symptoms, outside of that there is nothing else in the system which can really cause what you have as the selector switch controls almost everthing,try get a second hand one to see if it fixes the problem as not sure of the cost of a new one