easy mate, firstly, unplug the fan switch socket and get a piece of wire/metal and bridge the plug. If the fans switch on the its a dodgy fan switch confirmed. to put a switch in the car, tape up the old plug and make a permanant bridge using solder & suitable wire. Then seal to waterproof it. Make a break in the power supply to the fans at a accessable position and install a 12v switch, these are avaliable from maplin. Mount inside the car, job done