I have had several cars with mysterious water leaks. More times than not they ended up being front or rear window gaskets. I know, I also at first didn't beleive it. I spent time pulling out under dash areas, looking for blocked drain holes, finding open areas etc etc and replacing trunk gaskets etc.
Someone finally asked if the window gaskets were good. My reply was "Well they are not cracked or torn" Their reply was a simply test: Where the rubber meets the body, when the rubber edge of the gasket is lifted up, does it snap back? Mine didn't. They would kind of fall back. I replaced the gaskets and no more water problems.
Worth a try.
My 2 cents