There are only 4 choices. Hot or cold. Windshield defroster or the floor outlets. It's pretty primative but it gets the job done
My 73 has a defrost on off switch for the fan on the dash. It's a small 1/2 inch or so round knob. You open the two outside air vent knobs then turn the smaller fan knob on.
You are referring, no doubt, to the fresh air fan mounted behind the dash? That is a wholly separate issue, although I expect the idea behind it was that the fan would assist in drawing heated air more quickly up the defrost tubes. Not entirely sure, as the defrost heat tubes are installed independently from the electric fresh air fan. In any Super that I've owned, I've only ever drawn air with this fan directly from out doors... helpful on humid nights when one's windshield wants to fog up... but not much on actual defrosting (in my own humble experience). To push heat up to the front, the above instructions are what I find to work well enough. Driver's side up, passenger side down. But, I know what switch of which you are speaking, I believe sir.
Faithfully in Bugging,
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