After I spent 2 days fitting a new rear flor in mine, I got a build up of water very quickly in the channel just inside the rear door seal. I looked at the alpine lights, rear windows, I even went over the welds on the new floor with silicone. In the end it turned out to be the actual rear door seal itself.
I suffered this problem for weeks, giving my freshly welded rear interior a nice rusty coloured measles effect. I took the rear door seal off for some reason, cant remember why, and after refitting it, the leak cured itself.
Peter