In a way you were right anyway. A heater is just a small radiator.
As for on or off there are two trains of thought. One is to leave it on for the additional cooling it provides the engine although that puts more heat in the passenger compartment. The other is to put the valve on the "feed" side and shut it off during the summer. That helps keep heat out of the cab AND keeps pressure off of the heater core. It all depends on personal tastes and there is no "wrong" answer.
