Since the dealer is on summer holiday I tried to investigate the problem myself. Found out that it appears to be the rear wiper arm seal.
I removed the plastic cover from the inside from the trunk( 2 screws and 4 clips) so I had a full view on it. Then I simulated some rain I saw on the inside that the water is getting in along the wiper seal.
Its not much but its enough to slowly fill the very small edge just under the window, when thats full it just drops down.
It drops down either towards the plug hole in the center, so the water gets out(with the new plug) or in my case down to the boot look and spare wheel compartiment.
I'll give it a try and start with sealing out & inside wiper seal with loctite. Could also replace the rubber but as its the same part it maybe could happen again in a year or so I think.