If you remove the cowl cover plate over the cowl area, you will see that this area drains off to the sides, toward the fenders. Under the sheet metal area on the passenger's side, there is an intake for outside air. This intake is just slightly taller than the flat part under the cowl where it drains off.
I suspect that since you are parked at an incline (assuming nose end up), water is collecting enough against the firewall, and the back side of the outside air intake, to leak inside. I guess you could park nose end down and see if it still leaks....
Make sure you have good drainage (ie clear out the leaves and debris). While you are there, you should also apply some silcone sealant around the outside air intake, incase that seal is deteriorated. In my case, I had to cut out some of the sheet metal above the outside air intake to squeeze my hand in and apply silicone for a suspect leak around the outside air intake, I had on my '98. You can use some plastic sheet and silicone to cover up the hole you cut out. If you don't water has a direct route to your outside air intake.
My leak turned out to be the seal between the firewall and the blower box. Ample silcone on the foam seal cured my leak there...