The original speaker grilles (both front and rear) were re-covered with speaker cloth.

I used 3M spray adhesive to attach the cloth.

Related posts:

Fiero fiberglass headliner with custom upholsteryFiero fiberglass headliner Fiero PISA dash gap fix - Bondo filler to smooth out the surfacePrepping the Fiero dash for upholstery Pontiac Fiero PISA dash grilleFiero dash vent Fiero alarm system wiring is doneFiero alarm system wiring is done + more painting