Well without meaning to sound silly, you're just going to have to find where the water is getting in. The entry point can be quite tiny but really, it should not be too hard to find. If it's dripping down inside the wall from above there's probably a defect in the sealing of the roof. If you find a crack in the caulking or even a pinhole just seal it up with appropriate roof seal (not silicon). Obviously also check the window caulking and any nearby wall or roof penetrations (like ladder attachments, clearance lights, etc.). You should be able to find it. Also, check often even if there are no actual leaks. Good luck.
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