I wired a relay into the circuit , use a 30 amp relay and you should be fine , also use a larger wire 12g . I would go outside the harness except for the trigger wire for the relay . run the 12g wire from the starter to the relay and then 12g to the fan . the regular wire from your harness that would go to the fan can be the trigger wire into the relay and the last of the 4 wires into the relay would be a chassis ground . don't forget to ground the fan motor wire . by using a relay you are able to get more current flow than you can through a switch . Remember all factory cars run a cooling fan relay and large wires for a reason , they draw lots of current