OK, I've taken everything apart and I'm posting the findings for future readers. Refer to the attached picture from GM parts diagram.
The rear seatback is made up of 2 separate seatbacks 1-3-7 and 2-3-8 (upholstery, foam and shell). They are bolted together by tiebar 10 near the top and tiebar 11 near the bottom. Tiebar 11 has holes and is attached with 2 nuts to studs on each seatback. The nuts are removable from the trunk but not from the interior. Tiebar 10 has 2 tabs (heavy wire U shapes) that slide down into plastic pockets in the rear deck, identical to the ones on the bottom seat cushion.
Tiebar 11 is attached with a single stud and nut to each seatback. These are not accessible from the trunk, but they are accessible from the interior if you have small hands and use a ratcheting box end wrench. You slide your hand behind the seatback.
The middle panel 9 has tangs that hold it over the tiebars 10 and 11. The top panel 12 has 2 pins that go into panel 9.
The bottom of each seatback (7 and 8) is attached to the floor (actually to a bracket) with one large bolt.
REMOVAL WITH TRUNK ACCESS
If you can open the trunk, you remove the 4 nuts that hold part 10 to each seatback. Easily done from the trunk side. Then remove the bolts holding each seatback to the floor. Then remove the nut on one side of tiebar 11 and remove that side seatback. Now rotate part 11 down to free it from part 9. Take out the second seatback with tiebar 11 still attached to it. Part 9 will not come out as it's still trapped with a tang to part 10 and can't be slid up because of part 12. Part 12 can be removed at this point easier with the seatbacks out, but the procedure is the same as described below.
REMOVAL WITHOUT TRUNK ACCESS
This is a bit harder. You need to remove part 12 first. It has 7 spring wedge clips going straight down, so it is removed by lifting up. The middle is easy but the ends are trapped under pieces of quarter panel trim. The quarter panel trim pieces have diamond shaped tabs that go through part 12. You need to carefully lift those overlapping pieces with a trim tool (flat pry bar with a U shape cutout) untill you free the tabs from part 12. And hold them up, both ends. Now you lift up part 12 while avoiding re-engaging the end clips you just lifted out.
Remove the 2 big bolts holding the seatback to the floor. The entire seatback assembly can be lifted up and out in one piece. You're just freeing the tabs of part 10 from their pockets the deck.
Last edited by dxG6; 10-29-2018 at 05:30 PM.