I think that's probably the worst case of a previous owner not paying attention to what is happening with the car. I've never seen rust that bad in any MB trunk. It takes a long period of neglect to get to that stage.
Keep an eye on your trunk, replace that grommet, and be OK. Hope you can get it fixed without a second mortgage.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)