Try lubing the sliders. MB makes a special grease for this, but you could probably get away with using a light coating of standard grease. If that does not work you may need to tighten up the clutch in the sunroof motor gear box. Try a search, but don't make it too tight!

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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