3 areas that can cause this leak
1) The kick panel with the vent door thingy. replace that seal and make sure its TIGHT, it has to be REALLY tight for it not to leak. The seal is cheap enuff, but its trying to get that plastic kick panel tight without stripping any screws off.
2) Where the heater core hoses go thru the firewall. There is a shelf that goes at an angle on the firewall, its only 1/4 wide and about 3-4 inches long but when it rains , water runs down this lil shelf right into the opening where the heater hoses go thru the firewall. just get some kind of sealant and fill all that in so water can run into it. I had some stuff I use for cable, its like tacky insulation that you wrap around stuff, could get A/C or plumping wrap and build up that area around the heater hoses.
3) windsheld area and seal.