If you take out the tailgate trim, there is a flap of metal in each corner from the tailgate (which is easily visible). In my case, 1 side had dirt on it. It was also where water found its way in, and corroded the tailgate lock + connector.
I doublechecked by spraying some water on the bottom of the rear windscreen, in the crease between the window and body and then put some compressed air on it. The water went straight in
After repairing, it also solved exhaust gases coming into the cabin ever so slightly
