Rear left trunk quarter, what the heck I'd leaking...

[Bmwe5320023.0]

The image shows the path of the leak as far as I can see it, if I lay in the trunk and get my head in there.
This is driving me nuts, there's nothing to leak up there. These small drops accumulate and flood my amplifiers, causing lots of damage. I decided to finally get to the bottom of it and won't stop until I figure out what's leaking...
The quality is not the best, but I showed with the red line where the leak is, and on another picture circles a little water drop that has just formed.
Did anyone ever have a leak from that area?? Read countless threads, there's leaks from blocked drains, from tail lights, from water in the tailgate etc., but never seen leaking from above there....