on my 78 it had one wire from the switch to the fan...maybe re-wired from previose owner but not sure. It had a manual fan on a pulley and the electric only came on when hot.
on my 79 it had 2 coming off the switch. one to the fan and te other to the harness. there was no manual fan and the electric fan was always on while the motor was running and i think the switch was a back up to keep it on when hot after shutting off the motor.
reguardless i re wired all my fans. if you want it as a backup all you need is a fused non switched Battery power source ( I go sraight to the battery) and the radiator switch is the ground. if you want it to come on when the engine is running you must find a ground source that is switched possibly one of the wires you are looking at. or run a relay from the ignition to switch a groud source so that the fan runs while the ignition is on and stays on after shutting down if hot.
I would run a test light to the wires and see if one of them is a switched ground first.
my guess is the wires are fried and thats why some knucklehead wired it to the wiper motor.