Problem with 89 Grand Prix radiator fans

I have an 89 Pontiac Grand Prix 3.1L MFI V6. The problem I'm having is that the radiator fans don't kick on no matter how hot the car gets. However when you turn on the A/C both radiator fans come on and run fine. Is this likely a bad fan switch? and if so does anyone know where the fan switch is on an 89 Grand Prix, because I cant find anything at all about a fan switch in my chilton's manual. Or is it possibly a bad thermostat?

The sensor for the computer does double duty as the fan switch on your car. It is the one that is near the t-stat housing on the intake. There is a switch in the rear cylinder head that operates the guage, and that has no bearing on the fan operation. The fans are set to come on at around 230 degrees or so. I would check the wiring between the ECM and the fan relay, as it may be cut, burnt or shorted.