The fan uses a lot of current. Wouldn't you use a relay to turn on the fan? If you don't use a relay, you will probably burn out your ignition switch.

If you will be using a relay anyway, it's really simple to use a temperature switch to control the relay, and this will extend the life of your fan motor, since it won't be running all the time.

To keep it real simple, I guess you could put a heavy duty switch on your dashboard, but you'd have to watch your temperature gauge carefully!! Be sure your relay or your switch is rated for at least twice the number of amps that your fan uses. Don't use a horn relay for your fan motor. Horn relays aren't designed for continuous operation. Be sure to use a fuse in your fan motor circuit.