The last bit of repair on the c900 seat is the dreaded sagging seatback pocket. The other three 900s I've had never had this problem, but this one does. The fact that gloves were stuffed deep into one pocket and the heavy and large-ish owner's manual/paperwork 'purse' was stuck in the other one is probably why they sagged over time.
I've also seen this pocket-sag on older Mercedes and BMW cars, as well as the "other" Swedish brand which shall remain nameless.
I'd think the way I repaired this is applicable to any car that uses this type of pocket. If you have a 'mesh' pocket, like the legendary Mercedes W123 cars, you'll have to take a slightly different approach, since the elastic is exposed and not hidden under fabric.
This is the right side of the pocket. You can see the stitching that makes a little 'tunnel' flap that the elastic passes though, holding the pocket tight up against the seatback.
The pocket and the elastic were originally all sewn into a seam at the edge of the seatback. I'm just trying to undo enough to get at the end of the old elastic.
I'm going to attach a long piece of new elastic to this, then cut the old one on the side where it attaches to the seam on the seat - leaving a couple inches (maybe 5cm...my brain's Imperial-to-metric converter is working today) to work with to attach the new piece to.
Then I took a deep breath and cut the old piece.
It worked! In fact, it took 2 seconds for the thing to pass through. And to think I was wondering it the pins would get hung up. Worked like a charm.
As an aside: the elastic I used was 3/4 of an inch wide - it pretty much matched the width of the old piece exactly.
I cut the other side (left) and attached the new elastic to the short piece of the old elastic which was still sewn to the seat.
Then I pulled the other end of the elastic tight and fastened it to its respective old elastic end.
It took a few attempts to get it pulled up tight. I used small safety pins to hold the two ends of elastic together - I tried sewing them but it went nowhere fast. The thread just wouldn't hold together under tension. The pins aren't visible - they're tucked under the leather flap at the top of the pocket.
I didn't want to just cut the old stitching since I figured it might eventually unravel over its whole length. So I ran my stitches over it to lock it down.
The bottom of the pocket is pretty stretched out from years of having stuff in it. But overall, not bad.
Now I need to find time to get the other seat out and repair it.