DIY floor pans.

Ok guys im working late tonight lol i got the rest of the car cleaned out and all the old insulation out the drivers side looks solid all the way back to the trunk hump the passenger side is another story under the seat insulation had some surface rust i cleaned it with a wire brush and took a screwdriver to it really good i had one spot that went thru. What is the panel called behind the floor pan? Like the part under the seat? Its flat enough i can weld a panel in there im not worried about it. My question is, is why is the passenger side so bad? Where is all this moisture coming from only on one side of the car? Here are some more pics of the whole thing.

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