Interior Water Leak...

What about putting talcom powder around where you suspect the leak then taking a air hose and blowing from inside. Where you see the puff is where the leak could be.

Unfortunately it only takes a pinhole to let water drip in. Also, if you used anything but epoxy or a waterproof filler (such as the bond-o green w/ the short fibers) and left it unpainted, water will work it's way through the filler. If it's landing on the seat itself & not the floor boards, chances are it's at the top of the windshield. Lay some plastic sheet in the interior and try to see where it comes in using a cups of water...little by little in different sections of where the roof meets the windshield....may still be a pinhole up there in the channel. Check to see if they filled up to the molding clips, if not then I would try and seal around & up against their work w/ a good quality glass sealant & primer if you still have he moldings off the car. Good luck.