Further research has learned me that is for either rear defrost air inlet or speaker- one or the other. My car was luxuriously optioned with neither. I may use that opening for a speaker with a 1965ish grille/cover but the project at hand is the rear seat and carpet trim with their the 47 thousand screw holes under new carpet.
years ago I put a generic radio shack speaker in that spot in my 65 (so I could use a reverb system that I found in a chrysler 300). I didn't even use a grill or do any cutting. It sounded fine.
