What is up here? After heavy rains, there is a puddle on the passenger side floor. Is it something around the heater that needs sealed? How do I DIY?

 

I had a problem like that and still do but it hasnt rained hard lately to give me a puddle inside but on the passenger side I got a puddle after a harder rain storm and i thought it was coming from under the dash (and it might have been) but i have handles someone screwed on each side of the truck and found some water seeping in around the seat belts inside and figured it was that but i still think theres possibly a small leak around under the dash but i cant pinpoint the exact spot

 

I had the same problem with my '95. After much searching and pulling out my hair, I found it was coming in the back window. I pulled the back window out and resealed it and the leak stopped for awhile and then came back worse. After a hard rain I would get up to 2'' of water on the passenger floor. I finally found the leak in the body seams at the top rear of the cab above the window. I sealed it with some silicone and now no more water in the cab.