As water enters the cowl vent, on the driver's side, it runs over to the passenger side, under the wiper motor. Apparently there's a drain in the cowl chamber, to let the water out, but it will clog over time. Our '95 was getting a lot of water in the pass. side floor, and a previous owner made a mess with windshield sealant, thinking that leaky glass was the problem. I removed almost all the dash, and found rust streaks coming down from the foam gasket around the blower flange. I got a hose, and directed water into the cowl vent. As soon as water entered the cowl, it started weeping through the seam that I have circled. It then tracked down the firewall, and wicked in through the foam gasket, which at the time was only eight years old, and it acted like a sponge. Later on, my '92 (the one pictured) started doing the same thing. Knock on wood, but it's been seven years for the '95, and two years for the '92, and both have been dry ever since. Try the hose in the cowl trick, and see if you're having the same problem. And, give ehall's trick a try. These should work! Let us know.
Also, the factory used a lot of adhesive/sealant in the cowl area. Look around for any that is dry and cracked. Chip out any loose stuff, and replace it with silicone caulk.