How to modify a radiator fan?

Wire it to the ignition circuit..

I forgot to mention that it is a 94 Chevy Corsica with a 6 cylinder engine.

One way is to find an ignition wire that is "hot" when the key is "on" and wire it to the cooling fan relay, so that the relay is energized when the ignition is on. If it's like the 1989 Corsica V6, the cooling fan relay is controlled by the ECM (computer) which basically grounds the circuit to turn it on. You'll need to remove the 2 control circuit wires (not the POWER circuit wires) from the fan relay, run the ignition "hot" wire to one of the terminals, and run a wire from ground to the other terminal.

i think its not a good idea. I think you should hook it up the way it was designed to work. If it runs constantly you are placing excessive strain on your battery and altenator.

Not to mention wasting fuel.

And if the OP needs it to work this way, there is another problem that needs to be diagnosed.

the problem is he probably needs an ECM and on an old car like that it's not worth the money.

A friend had an 86? Cavalier with the same thing - the ECM controlled the fan and a replacement ECM would have doubled the value of the car, so he just wired up a toggle switch for the fan and watched the temp gauge. It was a beater, so that's an ok fix, but it's definitely not a proper switch - wire it up wrong and leave it on = dead battery... leave it on all the time = overcooling the motor, forget to turn it on=overheat. That's why it's a hack.

Ray