I live in a PNW forest, with tons of needles, pollen, and other crap on the truck every day. My 2001 gets water on the floor after heavy rains, and I have been working to clean up the door seal areas (mounds of consolidated gunk on the flat parts of the hinges and forward where you can see it comes down off the corner of the windshield. We'll see if that helps.

I'm suspecting that water is getting in through the ventilation intake just in front of the windshield. Is there an easy way to clear the drain there? I'm not really sure where it is, or how best to get at it.

 

Doors have drain holes at the bottom they do get clogged up, you can usually hear the water in the door sloshing around if you swing the door back and forth, clean the holes and drill more holes

Fresh air intake is on passenger side under cowl, cowl is not removable, its welded inplace, air intake sticks up above lower cowl "floor" so water can't get in unless water is at upper cowl vent level
There are two inspection plates on upper firewall in engine bay to service wipers, but also to clean out that area, one in corner on drivers side, the other in the center
There are drains on both sides under the cowl, water would come out behind front wheel on each side, just in front of the door

3rd Brake light on cab is a known leak point, and because most Rangers are higher in the back and lean towards drivers side the water tends to pool in drivers side floor

Windshield is mounted to a Ridge, and is sealed/glued to that ridge that runs around the opening
Around that ridge is a trough, that is where the windshield trim/moulding is installed
Water will get into that trough and will run down and sit at the bottom of the trough in the two lower corners
If the paint in these corners wears off and/or Rusts starts in the corners then water will leak in to the cab on that side, runs down behind kick panels, you can usually see signs of water trails if you remove kick panel

 

My 98 is leaking too, dripping down from above the parking brake pedal.Seems to be coming from a place behind the fender.. I ran water through the wiper trough area and it isn't coming from there or the windshield. I might try spraying undercoating with a hose in the area behind the fender.. I already took out the carpet so its not a big problem because there are drain plugs in the floor that I can take out to drain the water.