The heated rear window pane (compound glass) was an option. It is found on data card as #248. The switch is mounted on the inside of the wooden center tray, as shown in thread that Jonny B gave. A black box mounted at the head of the ash tray is at the correct location, but the switch mounting is not a box, and not black, it would be in color if interior leather/MB-Tex, with pull switch having white insert, your black box description does not sound original, and I would not know what it is for.
Bob