Pretty easy, get the right sort of wire for the job to start then...........i think you can, if your fan switch is working ok literally connect 2 wires to each on that and run them thru to the car to a switch so essentially ur bridging the fan but dont quote me on that!!

The way i had to do it as ive got an elec prob with it somewhere is wire a negative wire from the battery too the neg wire on the fans, then connect a wire to the posi side of batt, into the car to an on/off switch, then from the switch back to the engine bay and feed it to the posi wire going to the fan and hay presto.