Convertible speakers were low power shallow paper cone 6x9s mounted to the trunk floor in fiberboard (thick cardboard) boxes. The convertible top well was cut for grilles, but with the top folded down they may as well not be there at all.

I tried that first, before I installed mine in the piston covers. The problem is that "normal" 6x9 are too deep to fit in the factory cardboard box, and deeper boxes will take up space where the rear window folds down, potentially causing folds in the viny.

I installed Blaupunkt Overdrive 5x7, exceptionally shallow with good sound and synthetic cone. Did this in 2004, I am still happy with the results.

Another reasonable location would be in the armrest base.

That speaker housing shown is not a Firebird OEM part.