why would you want to this, just get a low temperature fan switch from bbpt and low temp thermostat if you wish to run cooler, but if its what you want. You can do it two ways. You could run wire to radiator fan switch or to the fan itself. For on fan put one to the earth and the other from a positve to your switch and back to fan. so it comes on and off with switch. Or you will find if you cross the connection that go to radiator fan switch (on radiator in front of alternator) the fan comes on, so you could just run a switch off this. Guage of wire should be fairly heavy, but in reality this type of switch should be done through a relay, so as to avoid heavy load on switch, but you would probably get away with it. If you tap into radiaor wiring, without disturbing original wiring, the fan would continue to work automatically, with switch being like an overide for putting it on.