One fan will run when the switch in the radiator reaches it's set point, and
the ignition is off.  Both fans will run when the switch in the radiator
reaches it's set point, and the ignition is on, or when the A/C and the
ignition are on.

What happens is the radiator switch either closes a path through the
resistor (with the ignition off), or closes a path through the cooling fan
relay (with the ignition on).  The cooling fan relay is also energized by
the A/C relay.  When the cooling fan relay operates, it bypasses the
resistor and applies 12 volts to both fans.

Also, the A/C relay inhibits the fresh air blower.