If it is a leaky seal (and not just some water carried in by other means) - by far the best thing for this type of problem is Krytox Liquid (DuPont GPL 105) which was first brought in to resolve the leaking seals issues on the roof of the VW EOS. I've used it, not for leaks, but on creaking door seals/rubbers/felts and it's a genius product.
You can order it from your local TPS (Trade Parts Specialists). It's pricy for a 30ml bottle (roughly £28 plus VAT) but a little goes a very long way so it lasts and lasts. Part number for the VW Krytox is G052172A1