Problem & Solution
Common among these chassis:
If only the bottom section is sagging down Kent's seat repair kit may be able to fix the problem. This innovative kit will allow you to beef up the spring strength without having to take the upholstery off the seat. (You will need to remove the seat from the car).
If the seat is totally shot you can try to fiind a good used one. You can actually swap out the seats between the W116 and W123 chassis (tracks are different though). Kent' manual explains how to do that. The original seats in the W116 were not as comfortable as the foam seats in the 1985 W123 so look at the picture above showing how Kent modified his 300SD to be more comfortable to drive.