Under Dash water leak

chances are it's the windshield. the correct way to seal it is with a urethane sealant. you'll need to remove the chrome trim, start by squeezing the end of the nozzle between the glass and gasket and filling all around the perimeter of the glass. next back fill the area on the outside of the car between the gasket and body. i know some people use silicone for this but ive been an auto glass tech for 9 years and i can tell you if you use silicone between the body and gasket and there is any bare metal (even scratches) the acid (vinegar) in the silicone will quickly cause rust issues. oh and gts340 is 100% correct about windshields being thinner then original, even on new cars we're finding the glass to be too thin when replaced and the moldings and cowl leave gaps between themselves and the glass. (damned outsourced production)