I'm not saying that it isn't the windshield gasket, but I've seen/read about many of these leaks. I've seen the windshield seal mentioned, but it has never turned out to be the source of the leak.
You need to check three areas. The first is underneath the passenger side hood hinge. There is a drain there between the horizontal sheet metal and the firewall. It is like a dimple in the horizontal piece. Use baling wire to clear this out.
Second look at for a similar drain underneath the battery and clear it out.
Third, remove the battery and battery tray. Check for rust anywhere underneath this area.
It is almost certain you will find the leak in one of these areas.