Water leak

It could be condensation from evaporator coil, when running defroster or A/C. You probably have a plugged drain. It is located near the center of the firewall under the hood and near the bottom. small tube coming out of the firewall. water should come out of this when engine is running with A/C or defroster running. If not it is plugged and you will need to run something in it to open. If it is flowwing good from there and you still get water inside you might have a leak in the heater box.

if the drains for the window cowling are pluged, you could get water backing up and running back through the heater inlet. look down in the vent area in front of the window. check for standing water. clean anny debris out of that area to help prevent blockage of drains. if you pour water on the glass it should runnout the drain behind the front tire were the fender an cab meet. hope that helps.