Cooling fan does not turn on?

The cooling fan o my 2000 Ford Focus 2.0 Zetec 16V DOHC does not turn on, but if i keep the A/C turned on this not happen. I think that the problem could be the temperature sensor, but I can't locate it on my engine, please can somebody help me with it, my car overheat more than once, i need to fix it. Any information would be greatly appreciated. Please answer me on edinpop@gmail.com, Thanks so much.