Water soaks passenger side floorboard only when rains, frog

1999 Mazda B2500 with 41,200 miles. Water soaks and fills the passenger side floorboard when it rains and the truck is not running. Seems to be coming from under hood behind glove box, but can not really locate the exact spot with water hose. Please help. Just sold it to friend and when he went to work and it rained, the passenger side was full of water. It was my Dad's truck and has been sitting under a car port for 8 years and not driven much and certainly not when it rained.

Answer
    There are only two places that the water can get in.  One is the cowl vent that supplies air to the heater/AC.  Water can normally get in, but there is a drain in the bottom of the unit which allows water to exit through the firewall and down onto the ground.  This drain is usually fitted with a little rubber spout, and is sometimes clogged with debris from trees and so forth.  Since your truck was under cover for so long, this is doubtful.  A mouse, however, could get in there through the spout and build a nice clogging nest.  Run some hose water into the vent and look at the drain to see if it flows out freely.  If it doesn't, then that needs a thorough cleaning out (use a coat hanger with one end bent into a small hook).  The other place it could leak is the gasket around the windshield.  Sometimes this is hard to diagnose because it needs a real frog-strangler to get the leak started.  Buy some clear silicone sealer at the auto parts store and run a bead around the inside and outside edge of the gasket.  This should seal it up.  It is possible that the door could be leaking into the passenger compartment.  Give the door gasket a good looking over.  If it is the door you should be able to see the obvious fault in the gasket.
Car Repair Talk's forum.