72 Dart Swinger Rear Window Tray openings?

Pretty sure they were defrost vent holes.
nope.
rear air (defroster/defogger) "vent" on all mopars of this era is the long - rounded corner - slot located dead center of the cars rear shelf. That housed the black plastic duct & top trim. blower was mounted under side of the drivers side speak hole, with an accordian fabric tube connecting the two. applicable wiring to dash switch on left side.

There was a 2nd style ( self contained type) used on some 1974-76 plymouth brougham 4 doors and the applicable dodge model as well as a few B-body 4dr and F-body cars (aspen / volare). that entire unit mounted within the confines of 1 speaker hole, no tube or duct and had a top cover that looked like a square speaker grid which almost never survive due to UV making them dust after a few decades.
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