I have the same problem. After heavy rainfall I get patches of condensation on both the rear windows, same location as your pic. As you pointed out, the rubber seal between window and door looks to have broken down over time and actually looks to have perished completely at the corners. I’m driving an ‘06 GTi
Presume moisture is creeping inside the door frame due to the perished seal. Would be interested to hear of any fix. Can the rubber seals be replaced??
Cheers,
Stewart