Ok, I pulled apart a 560SEL at the junkyard, and I now see how the seat comes out...boy I feel dumb now lol.

Anyway, I have the thumping noise while going over bumps, and I'm not sure what part exactly needs to be replaced. Can someone pick out the part from this diagram??

http://www.performanceproducts4benz.com/diagram.asp?diagram%5Fid=107

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles