An easier test is to not put it back in the car after the freezer, but just hook some wires with alligator clips to the AAR terminals and a car battery. That way, you can see the flap close (and feel the heater working in your hand). It's also handy for testing the AAR. When the flap is fully closed, try to blow through the AAR. If you can, it's no good. My experience with these things is that they are almost all bad on older cars. I had two spares from parts cars that I tested before replacing the bad one on my '76, and they were both bad. So I bought like 5 or 6 of them at half-price day at Pick Your Part and all but one tested bad — that is to say, you could blow air through them when the flap was closed. The only one that worked, and which is on the car now, was a slightly different design out of a 280ZX.
Mpcapps is also correct that most AARs are also heated with a bypass line from the water jacket going to a little 'heater' under the AAR. The only year this is NOT done is on the first-year fuelies, the '75 280Zs.