Ok. I pulled the seat out of my 1995 500sl. I should have taken some pictures to post what the seat looks like when you pull it apart, but I was kind of angry for paying a upholstery guy 150 dollars to make my seat even more uncomfortable than it was.

To take the seat cover off the metal frame I turned the seat leather down on a table and pushed down on the metal frame edge toward the table. The leather seats have plastic strips sewn into the edges of the leather. Massaging the leather from the sides of the seats upward the plastic strips started to slip out of the metal retaining groves. I used a screwdriver and slowly pried my way around the circumference of the seat.
Once the seat was removed I could see that the leather is backed by a 1 1/2 in. thick piece of mercedes matting and the pleats of the seat were being held down by 3 sets of wires running from front to back of the seat. I snipped the center pair and removed them and restuffed the seat with some 1/2 in firm foam. I also stuffed the edges of the seat with more foam. I am 6'2" and 260lbs and I think I was bottoming out on the wires. After 40 min. in the car I was numb. This procedure helped quite a bit, but I still find the seat geometry pretty uncomfortable for long drives.