that switch on the ac line controls the coolant fans so they run when ac is turned on , that harness for hi low makes use of that to active the cooling fan relays
With stock wiring the fan comes on high for 3 things. 1- AC pressure above 300 PSI. 2-coolant fan delay relay after the car is turned off. 3-temperature above 226 deg. when the coolant temp switch comes on.