Hey Dee8go,
I stitched my back seats to fix the ripped out ribs.
I used heavy-duty, waxed upholstery thread with a tapestry needle and a thimble to save my finger tip when pushing the needle through the MB-Tex. The thread was similar in color to the seat cover and I used the existing stitching holes in the cover to sew the new thread into and hold the new muslin under the ribs.
If you end up sewing your seat covers to repair the ribs, Use some spray adhesive to hold the felt ribs and the muslin in place while you sew. It makes it much easier to get the right spacing for the ribs too.
Here's a thread with pics:
Fixing the Rear Seat in a 300TD