I have what appears to be a common problem with these cars - water in the rear passenger side footwell.

Has anyone found the definite culprit?

"Definite" is difficult, especially from the far side of the Internet, but likely: Door seals.

Door seals ^^^^ or plugged drain at the pillar.

Blocked sunroof drain tube, or compromised weather seal inside the door trim panel.

Do you have a sunroof? If you do, open it to a position that lets you see the drain openings in all four corners of the sunroof tray. Pour some water into the rear passenger drain, and observe the ground under the car to see where the water comes out. If it comes out as fast as you pour it in, the drain is not plugged. If it doesn't come out, you found your water problem. If the drain is plugged, do not try to blow it out with compressed air. That could cause the drain hose to disconnect, and now you will have a wet headliner. IIRC, the outlet is behind the rear wheel fender liner. Pull the wheel liner and see if he outlet is plugged. If that does not work, you may have to drop the headliner and remove the C pillar to see where the blockage is.

If you don't have a sunroof or the drain is not plugged, the next likely candidate is the plastic weather seal inside the door panel. Remove the door trim panel and you will see a sheet of translucent plastic stuck to the door with butyl tape. If the seal is pulled away from the tape or the tape is compromised, water will leak into the rear footwell. If it looks like this is the problem, remove the plastic sheet, clean off all the butyl (nasty, sticky stuff), reapply butyl and stick the plastic back on. You can get butyl tape at most car part stores, Amazon or Ebay. You want a 1/4" bead. Be careful to follow the original butyl path.