I purchased these bolsters to fix the seat sag in my 1985 300TD.
You don't need to remove the seat, just compress the seat bottom and slip the seat skin edge band out of the retaining lip. Then pull the skin up and over the seat for access to the springs.
Here you can see how the bolsters fit inside the springs.
I also noticed a broken spring so I took a piece of old hydraulic line and put a piece of metal hanger inside it to make it stronger. Then I bent it 90˚ and wrapped it with bailing wire along the broken area of the spring.
After that I pulled the seat skin down and compressed the seat bottom so I could put the edge band back into the retaining lip.
I did both front seats and they feel like new seats again!!
Both seats took me less than an hour.