For a basic electric fan, I would power the fan with a relay, usually 30 amp minimum, continous duty. Check the fan specs
It can be controlled by a simple switch to the relay, or best done by using a thermostatic switch. Not sure what vehicle you are installing it on. There are fan control kits that use sensors that just mount in the air stream on the back of the radiator, and those that use switches that mount in the cooling system. A 2.3 has 3 in it already, one for the EEC, one for the gage, and one for the orig electric fan controller. Some run these hot all the time so the fan runs until the system is cool, but I would power the switch from a keyed source.
The temp sensor in the coolant system is best, and can be installed in parallel with a manual switch for over ride, and or for activation when the ac is on.
I would not power the fan directly with an switch, because the length of wire is such that you will need 10 or 8 ga wire and a 30 amp or heavier switch.
