Strange one this!
If you push the right hand heater control up (normally the fan would start on low speed) mine is starting the air con fan instead! But the air con light is not coming on.
If you move the fan speed switch the normal heater fan does not come on atall.
If you have the heater control up so the air con is working, pressing the proper air con button does not stop it, but you can hear a solonoid type click when you turn it on and off from behind the glovebox area.

Any ideas?!

(It worked perfectly last week!)

 

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