I've always had the same complaint with my '91 300CE.

I took it in to a seat shop for re-stuffing, but I felt little difference.
What worked for me is one of those lumbar wedges made of foam rubber. I don't have or have ever had a bad back, so I was skeptical when it was suggested to me. It did work.

Try it. If it doesn't work, you're only out $10.

Phil