THE A/C FAN WILL NOT RUN ALL THE TIME HOWEVER, IF YOU TAKE A SCREW DRIVER AND HIT THE BACK OF IT, IT WILL COME ON BUT, THEN IT GOES OUT AGAIN. WHAT COULD THE PROBLEM BE.
What I'd do (and I ain't suggesting its correct) is to first check the connector which is near the fan - do this when its not working but should be - and wobble it a bit - if fan comes on the the connector is the problem - next I'd saturate the fan in WD40 or similar either in-situ or removed.
There is a relay for the cooling fan somewhere...maybe under the battery??? also check the fuses on top of the battery.