I would suspect the '65 and '66 fan switches would be the same, as the instrument panels were similar.
In those years, there were two 6x9 rear speaker holes in the rear package tray. One was for the rear speaker option and the other one was for the rear defroster grille. Now, if the rear package tray trim was for the rear speaker option, the slots for the speaker should be on both sides, as it was on other C-bodies that came with the rear speaker option. All it will take is to take a knife and cut through the perforations in the just insulation under the package tray trim panel, remove it, tidy-up any "hanging fabric", and install what you desire in that location, on the bottom side of the panel. I don't recall if there's a rear heater core for that option or if it just blows air? Air intake in under the rear seat cushion, I believe? The factory service manual details this stuff.
Here's what I'd consider . . . upgrade the alternator to a more recent 100+ amp model (think 100amp model from a '75-78 Cordoba with the rear window defroster option) and then get the aftermarket accessory seat heater panels and put them under the seat cushion upholstery already there. Then put two 6x9 rear speakers on the rear deck. If the car has factory a/c, use that until the heater gets hot to dehumidify the air in the car, then use the regular heater when the coolant gets hot.
As far as the hardware for the rear defogger option, I suspect it'll be very similar for the '65-'68 model years. Might be able to adapt the '69-'73 cars' items, too, other than the correct model year switch and mounting items in the instrument panel.
Good luck,
CBODY67