after i made my comment, i looked closer at the FSM, and although i couldn't find mention about the rear defrost, i could clearly see the mounting surface for it on the package tray picture.
as said, it was clearly on the left side, with [of course] the vent outlet in the center.
even though this was a 1970, i took a peak at my charger, which is a 69, and it was [basically] the same. same [basic] cut out and mounting surface, and same [basic] vent shape and location.
this prompted me to do a little [snooping ?] investigating, and the A-bodies of my buddy's 71 duster and 73 dodge sport are somewhat the same in the package tray area. it looks like a defrost assembly could be mounted in the [somewhat] same location, and the vent used in the middle.
now with that said, is it possible defrost units were mounted to the left on all body styles that had this as an option ?
if so, how does this affect rear AC units installed on new yorkers and imperials ?
remember, i'm just throwing this out there, and although i have owned imperials with rear AC, [plus scrapped some in my junkyard ownership days] it's been so long ago, and i don't have any FSM's to refer to.