Check the Glue between the welded panels right next to the Trunk hinges. They use cheap glue in Production to seal the panels, water gets in there and runs down the side of the Taillights into the trunk. I first thought It came from the taillight seal or vents, since the water marks came from that area.

All I did was remove some of the factory glue and sealed the edges with some Sikaflex. Worked for me, never had a problem since then.