Check the joint between the body panels, where the hood cowl connects to the windshield area. I can't think of a better way to decribe it. But anyway the seam should have seam sealer in it, after 15-20 years it get old and falls out, now water can enter that seam and ends up under the dash all over the floor. Before I repainted mine I dug out all the old stuff with a knife and replaced it, no more leak. Also check that there are no open holes on the top of the windshield frame from old soft top installs, and if the windshield hinges have ever been removed the sealer around the nuts on the bottom half of the hinge keeps the water out, if it is gone you could have water coming in behind the hinges, though the holes and on the floor.