I haven't done the rears on my G6, so I can't speak with great certainty, but I have done several other cars including GM w-bodies. On those, you remove the rear seat bottom, then the seat back, then the trim on the C-pillars. After that, you can remove the rear package tray to access the speakers. It sounds worse than it is - the pieces tend to come out without too much difficulty.
If you cut out the openings, you'll probably regret it later.
HTH.
Doug
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