On '71's or other early C3's, where is the control for the rear defogger/defroster option? How can I tell if a car has the option or not and how does the air get routed to the rear window?

My 75 has a rocker switch (high/low/off) on the small trim panel on the shift console. The defog blower grabs air directly into the eye of the fan, and then delivers the air VIA a flexible duct to a grille on the left side of the window. Hope this helps, I would be surprised if the older ones were any different.