you could do it this way and its much easier,go to the fan switch on the side of the rad,on the connector there will be either one or two brown wires (earth) and another colour wire(not sure of the colour) which be should be a permanant live.simply splice into the permanant live wire and run the spliced in wire to a switch on your dash then earth the other side of the switch,
all you are doing is manually earthing the rad switch,you should have one fan opperate with ignition off and both fans with ignition on,although this may not always be the case