I am no way a Auto Computer Expert but I think that adding a Resistor may be the simple route but may cause more problems. I beleive the computer controls alot of other items in the car based on the temp. Things such as Air/Fuel Mixture and others. You would have to put the resistor on the Temp sender unit which sends the current temp to the computer for fan and other things. The only correct way that I would think is to have a Ford Dealer reprogram the computer internally to start the fan ealier. I think I had heard some one on this group say that Ford did it for them but that was a long time ago.
Thanks.
Patel