Had this problem an both my 91 and 92. Right under the windshield wipers is a vent cover held down by about seven screws. To keep water from leaking through the screws there is a rubber boot placed in the screw hole. the problem is after time the boot dries out and cracks letting water through into the vent and onto the floor under the glove box.
on the Outside.
I pulled the wiper cowl off, and then the other half cover off. Cleaned everything off and filled those boots with silicone. Then put silicone on the bottom edge of the half cover and put that back on and put the screws back in the holes that are filled with silicone. Then I put some silicone around the edge of the half cover. Also put some around the other clips in that area, just in case they were leaking too. Let it dry a bit and put the wiper cowl back on.
Now onto the inside.
I took out the lower dash panel and the metal piece behind it. Took out the fanassembly, and cleand/dried it out. If the water is leaking in from above, it leaks into the fan, so you want to clean it so it doesn't get mold/mildew and start to stink like smelly feet/piss/ass. You might want to get a spray bottle and put some water in it with some bleach. Spray the ac condenser (the black box next to where the fan was, looks like a little radiator inside) spary the radiator looking thing. Just to kill the bacteria, if your car smells that is. There is a tube that the water will leak out of. if you look under your car at the fire wall in tha area you will see it. You will want to check to see that is not clogged. Now that every thing is dry. Put the fan back up. Now you don't have to do this but I did just as a back up in case water still got in somehow. When the water comes in, it goes into the fan, then from there into the condenser box. But on it's way to the condenser box it leaks out from between the two. So I put silicone between the the fan and condenser box gap. Had to put some on my finger and wipe it up on the back side a few times. Then I put some on the bracket and put the bracket back on. Then I put some on the edges/sides of the bracket.
Car hasn't leaked since. Been a few months. And since then, we have had tons of rain, and even some snow. And car doesn't smell like feet/piss/sh*t when you turn the ac/heat on anymore.