Two ways I know of are 1) the one most previous people talked about - tapping into the computer, and 2) running the fan directly. I went direct since I believe tapping the computer requires the key to be on. I want total control so I can cool between races (of course you need an electric fan for that too). There is a 75 amp Bosch relay many on here have used. Do a search or I will get the number if you cant find it. The relay lets you keep the large amperage wires required for the fan in the engine compartment, so you can mount the relay near the fan. The switch only controls the relay with a low amp signal, so light gauge wires run from it to the relay. The switch wires as follows: use a 12v signal from a source in your fuse box for the power, ground to chassis, and 12v out to the relay in the engine compartment. Use a voltmeter to find a constant 12v source at the fuse box. The relay has 4 connections: signal from switch, ground for signal, power source for fan, power out for fan. Long answer, so write back if you need clarification.
'94 GT w/306
ATI D1-R, Powerglide, 4.30's