If you are sure that your door seals make good contact with the body all the way around, I'd check the fresh air vent area then the windshield rubber. If you have a sunroof, make sure that the two small drain holes which feed to the area just forward of the engine cover are not clogged. Porsche floors are the perfect trap for any moisture. One of the best insurance policies I've found is installing a few of the small rubber drain flappers that Mercedes used in most of its cars. They allow water to drain, but not to come back upstream. As for sealants, I've never had much luck with them, as it's difficult to get a perfectly even seal that flexes like the rubber. Then there's the old trick, which I'll post here for curiosity's sake & not to be taken as advice. A little Girling brake fluid applied to rubber weather seals with a Q-tip will swell them to new, oversized dimensions. As we all know, it'll do the same to a beautiful paint job.
