On the back of the thermostat housing is a one wire temp switch that connects to ground to turn on the fans at full 12 volts. You could put it or any other temp switch from auto zone of your temp preference in the hot side or cold side of the IC and use it to connect the ground circuit and flip the fans on. I have a wire running off the floor of the car to a econ/pwr switch in the center console then up to the firewall and through that goes to the single wire that use to go to the factory switch. That's probably the easiest way to wire it up. As for on and off temp of the switch the factory one is like an oh crap 225 degrees or something, but any factory one wire should work but for fine tuning will take a bunch of research, I think auto zone has a book that has pictures with on temps listed but its a picture book and will take some page turning to find the range you want. Make sure the one you getis a sealed unit and the mercury is installed through the outside/wire side so if it gets to hot and pops it doesn't fire the end into the IC system or on into your engine. Some are pressed together and I've pulled them from cars before and parts be missing.
