It's probably going to be the rear applique. It was replaced on mine for the same issue. The problem is the seal shrinks a little bit, with age and can let water in, if it's coming at just the right angle.
As for the remark of "Guarantee Porsche, Mercedes and BMW don't treat their customers like this," you should just do a little web search for horror stories, causing greater mental anguish to other owners. The experience I had with the MB that my Tesla replaced, makes your experience seem like a rather pleasant interaction.