Mystery water intrusion

So you're talking about in the back seat on the right where the passenger sitting in the seat feet would be ?? ??? If you look underneath the car you'll see a big round hole where a rubber plug should be installed with the carpet pulled back from up top . If you don't see that rubber plug then you're looking at the back side of your carpet not goidchor puddles

 

you've siliconed the rain gutters under the trim?

 

Check that the drains on the passenger side rear door are not blocked. (Probably not the problem, but slight chance it could be.)

Does the car have a sunroof? If so, start with that. There was no OEM, if there is one, a dealer or a previous owner put it in. Verify that the drain hoses in all 4 corners are attached and that they are clear to where the water leaves the car in the wheel wells. Check that the glass fits to the rubber seal tightly all the way around, and that there is no damage to the seal.

Finding the source of a generic leak is a PITA. In this case I would start by pulling up the carpet as much as possible on the passenger side. Tape bits of paper towel down all the way around. You might not see a few drips go by, but if they pass over a paper towel it will stay wet. Then with all doors shut, and an observer in the car, go around the lower part of the car on that side with a hose and spray it gently. (Like rain, don't blast water in through the seals with pressure). Work your way up SLOWLY, doing one section at a time, until water appears. It may be necessary to remove the ceiling liner and the A,B, and C pillar moldings if the leak doesn't happen until the water is sprayed high.