Pictures would help, but you will need to replace the trailing arm if it is that rusty.

You may want to check out the rest of the body for rust problems and re-evaluate before you go too much farther.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine