Try this :
Step 1 : connect direct to the fan from the battery ( be careful, don't short anything out and make sure you have + to + and - to - ). If the fan screams like a banshee, then it is OK.

Step 2: borrow a FCM ( Fan Control Module - mounted on the driver side fender well ) from a friend,
or buy one ( Crossfire or Mercedes SLK320 ) from a junk yard ( car-part.com ). See if that fixes the problem.

Step 3: ohm out all the wiring from the PTCM to the FCM and FCM to fan.

Step 4: doubt if you get here, but you might really have a bad PTCM.