I had the same problem in my w201, the driver seat had two of the four springs on the left broken!

first I tried to fix the problem, but I realized that the car was extremely uncomfortable for a longer drive, my back and my left leg would start to hurt really bad.

I switched the driver seat with the passanger seat, just the bottom parts, and so far so good, I still had to adjust it a bit to get as much comfort out of it as possible, and I have to say that this area in an MB could use some improvment.

Nevertheless, I don't notice it anymore and I'm able to drive the car without any problems.

xp

Well, my piece of scrap wood is still in place. Makes a huge difference. I actually sit up higher in the car. I haven't been in the car for more than 15 minutes consecutively, but previously on hour long rides my lower back would fatigue. I hope I have found a solution, albeit jerry-rigged. Hey, you can't see it, so what the heck.

I had the same problem with my 300D. Talked to a trim man and he told me to get some foam padding about 6" thick from an upholstery shop and wedge it in the area where it is sagging. Worked pretty good for me as the seat is alot firmer now.
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80 450sl
85 300D
97 E320