water intrusion, passenger side

[79 stepside]

I have a horribly rusted out passenger floor pan thanks to a leak somewhere on the passenger side of the truck. ('79 stepside, duh) I have had the windshield resealed. Wasn't coming from there. I can pour water into the openings in front of the windshield, and that water makes it into the cab. Sometimes it looks like it's coming from the side, sometimes from the middle of the footwell area from behind ductwork. I figured the seam had been compromised so I cleaned it up and sprayed a rubber sealing type compound all over in there. Still have the leak, and can see the ground through the floor.
Didn't find a search function so I just asked. I hope this is common and someone has an easy answer.
Thanks yall,
Jimmy