Didn't the convertibles(at least 67-68) have 6X9 in the rear? That is what the Year one catalog shows. I just ordered new speakers and mounts from them. It appears that they attach to the metal right behind the rear seat. My car originally only had AM and the dash speaker.
1967 400 Convertible 1978 Firebird 1980 Trans Am Pace Car (Sold) 2002 Ram Air Convertible (Sold) Mike