This is a problem on my SL as well. I had the seat cushions out to inspect, and the incredibly solid metal frame looks fine. So I have yet to find the real problem.

I should have considered that the previous owner may have been quite large...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car