You can see the effect in my previous post about the front seat.
Even had we known about the sewn-in padding, it would have been very difficult and maybe not even possible to do with the sewing machine in use. (We are using upholstery thread and the sewing machine is coping but there is not much reserve.)
The rear seat-back before stripping.
This is the back of the rear seat upright part (shown above).
Note the method of holding the vinyl in place in this 'before' picture. They had some kind of wire tool that stapled through the vinyl or foam.
First we covered the whole back of the seat with 10mm "firm" foam (blue). Yes those are the same clips that held the headlining during glueing.
The base of the rear seat has 35mm thick foam which originally poked out past the frame by about 60mm. The vinyl wrapped down around the foam and held it in position.
We can't do that with the fabric, so the 35mm foam now only protudes about 5mm past the edge of the frame and we have glued 20mm foam to the edge of the 35mm foam.
The base of the rear seat frame before painting.
Foam for front seats waiting patiently in our bedroom.