My 2013 Tuscan has a leak in the rear tailgate. I've sealed the window completely but it still leaks. Any suggestions?

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Every time it rains, I have water leaking through the plastic molding inside the tailgate. Water also collects in the spare tire well below. I've tried sealing around the rear window and tail lights but the leak hasn't stopped. Any suggetions? Thanks, Mark What seems to make the problem better or worse? Its worse when it rains How long have you had this problem? Several months