Does your defog work when it's cold? Cold air (with you inside breathing warm moist air) is not efficient at defogging a window. Dryer air works better. My parents Olds, I know it clicks on.

I've got a Ford Tarus now and I bought it in becuase the windows were down on a cool day, and even with warmed air blowing in, the compressor wouldn't shut off. I thought it was broken. According to the dealer (who might be lying), that's how it's designed. I wasn't happy with that, but at the same time not upset enough to wire in a switch.

Obviously, different experiences with different cars means they're set up differently. Given the new Rodeo (not much of a data point), I think Japanese manufacturers still keep the A/C (compressor) button in, and assume you are smarter, while American car companies assume you are dumber. Age of the vehicles is also a factor--your last four cars; were they all from the last three or four years? They might also vary it by model. A newer, more expensive vehicle with a digital temperature setting will probably have a fully computer controlled compressor, while at the same time running the compressor no matter what when the defog is on.