The webbing in the seat back goes bad the same as the that in the squab excepts it is mostly due to age rather than age and bodyweight. Not so easy to get the seat back cover off so it rarely gets renewed. Once you've got the hog rings out of the bottom edge it's down to a lot of patient pulling once it's on the bench. An extra pair hands is useful. Repair/replace/re glue the sponge if needed while you are in there. Replacing it can be even more trying but you can cheat and use a thin plastic bag from a swivel top kitchen rubbish bin with a bit of silicone spray or Mr Sheen on the outside to help slide the cover on. You can either leave that in or yank it all or most of it out afterwards. Hog rings can be replaced with suitable coloured mini cable ties if you're not desperate for originality. Do it neat and no one will notice. Good luck.
