Since it's on tthe driver side, I'd look at two area's. One is the seal under the windshield. Water could be getting in there and leaking down. The other would be the cowl seam. I don't know why it's just bolted together then sealed rather then welded. The calk shrinks over the years and allows water to leak in. You can look at the seam from outside. Look for the seam at the bottom of the vertical part of the windsheild support and the cowl/hood.
Other places it could be leaking from would be the vent (if the drain clogs it could be overflowing. The other area would be the windsheild frame itself. Although I think these would be less likely.
Try looking at the back of the dash. If it's been leaking for a while, you should be able to see rust.
