The rear package tray sheetmetal is already stamped with holes for 6x9 rear speakers. No cutting is required. Remove the back seat, slide the fiberboard package tray out, install your speakers, and reinstall everything. Heres the rear deck on my sons 69 cuda. The package tray is already stamped for 6x9 speakers. One at either end. The 2 round holes in the middle were for a rear window defogger. Package tray even has pilot holes stamped in for mounting the speakers.
Upsize the pilot holes then stick your mounting screws through from the top then lock washer and nut them on the bottom sandwiching the speaker deck package tray sheetmetal in the middle. Now you have mounting studs, and you fit the speakers in from the bottom up. This way if you have to change one out you dont have to remove the fiberboard package tray to get them out. If your fiberboard package tray already has speaker hole slots in it, pitch the cheezy plastic speaker grille covers and slip it back in place. Or get one that's got the slots in it and hide your speakers
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