Thanks for the pics.

The 126 and 140 are different in that the seat back upholstery doesn't tuck into the cover. After the screws are out, very little tugging will break the cover loose. No need to compress the seat back to slacken the upholstery or mess with clips or anything like that.

There's a soft spot in the cover that aligns with the headrest riser clip when the headrest is in the fully extended position. No need to remove the cover to release the headrest. I'm sure you know that - just making it clear for the archives.

The part that usually breaks is the track and runner. You can see these plastic parts at the end of the flex cable from the motor in pic 5. The track has two tabs that extend through the vertical metal headrest frame. Tap the metal retainer upward about 3mm and it will disengage from the tabs. The retainer has a couple of keyhole cutouts that engage the tabs. Easier to see how it works after you remove the retainer. If you catch a crack where the mechanism clasps the bottom of the headrest post early enough, you can reinforce it with bailing wire. You have to replace it once pieces fall off.

I'm not that big and thankfully still flexible so I have no problem servicing the seat in the car. I miss the 126 and 140 because there's room for a couple of kids in the back seat with me as I work on the seat

Sixto
87 300D