last year i removed the quarter panel to install a switch inside the rear passenger-side ashtray to control the antenna
on the rare occasions when it needs to be up.
panel removal is straightforward.
mine came out without anything breaking & without reference to Factory Manual.
my dim recall is there may be a screw under the ashtray, but that could be another car...
retainers are old-style: they pull out and push back into round holes.
the gm J 38778 tool to remove retainers looks like this:
after removing seat and seatBack, follow these directions from the Factory Service Manual: