My guess would be, knowing how seats are often tensioned with stiff wire, something as follows:
1. The dark plastic section pushes lugs through the leather.
2. The clear plastic strip is inside the fabric piece which acts as a sort of sleeve.
3. A metal tensioner strip passes through one row of holes in the fabric (probably the two long, two short).
4. The lugs pass though the 4 smaller holes and through the clear plastic strip, probably secured there by small metal clips (starlock?).
When assembled in the factory the tension is applied as the cover is half-on before rolling it all the way down.
So you may be stuck with a "soggy seat back" for ever! 
Edit: Ah, if your seat covers have a removeable back piece you may be able to get at it!
The S8 Recaro covers are all in one piece so getting inside them is next to impossible! (As I ideally need to to replace the lumbar pumps!)
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions, ski hatch retrofit; extended leather. Aftermarket DVB-T, reversing camera and full XCarlink (Bluetooth etc.).
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation".  (A much rarer model than a D2 S8 by the way!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. AgateGrey/Platinum. Every option (I think) except electric rear seats, tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon, Auto-dim mirror, Leather, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
Last edited by HPsauce; 10th March 2019 at 11:29 AM.
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