I'd recommend you look at the scuttle panel to see if the drain holes are free from foliage. If they're blocked then water can build up and leak through the bulkhead.
In addition, you could look, at the windscreen as failed seals/glue etc can fail. Especially if it's not the original windscreen.
Also, is the car loosing any coolant?
Or the heater not working properly? I ask as a failing/leaking heater matrix could be the issue.