my dad has a 1997 3500 ext. cab and every time it rains the passenger side floor board is soaked any idea where the water comes from

 

Look for a rubber hose that drains the AC condensation and rain water out of the heater. It will be under the heater and extends down outside the firewall. Clean that hose. When you get that done, stop by an AC shop and pick up a bottle of the foam that you shoot in the system and kills all the mold that plugs that hose.

 

Or, since it's happening when it's raining, check the gasket around the 3rd brake light.


phox

 

Gasket at 3rd brake light.
Glue seal or gasket around back window.
Glue seal around windshield.
Weatherstripping around door (check for wicking under the U channel).
Screws that attach the outer weatherstrip above the door opening.
Weatherstrip above door opening missing or out of position.
Seams in the firewall.

 

Mine had a bad hinge, replaced and adjusted it, no more leak.

 

On mine the weatherstripping on the door either slid back or shrunk so that there was a gap about 3 inches long up in the corner by the windshield. I slid it forward and used a flat-bladed screwdriver to "crimp" the channel that the strip slides in at the back side. No more issues.