Now, have removed the sill panel at the driver’s side today (to expand the fender). And although sometimes nothing could recognize at the edges of this panel, the sill panel at a corner (back) is totally marode. Could pierce with a screwdriver so through. Also the fender itself was totally rusty at the underside, where it is covered by the panel. I think it is a real flaw and we are lucky that the Tüv examiners do not remove the sill panel. Who dares to remove the panel and controls his elk there? Simply put it well in the underfloor protection. I guess it is there with many marode/rusted., perfectly disguised by underbody protection (so a kind of rubber layer) and just this paneling. The panel itself is to blame. Behind the panel is a kind of water film, which only slowly dries out. Keyword: cappilar effect.