Have you had the door panel off? There is a bitumen mat which is a fairly poor attempt at a vapour barrier. That might be torn or loose, or even missing. Alternatively, does the door fit in the frame properly? If the window frame is misaligned it might not seal against the rubber properly and allow rain to drip in. Is the door card damp?
Maybe sit in there with a torch while someone sprays it with a hose and see if you can see a drip?
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