Hey Raz, just wanted to add to this one because I just had to deal with similar issue in shop. This is a some information from another Grand Prix owner that posted this on the web, I found to be helpful:
I also had the same problem and spent over two thousand dollars before I found out what the problem was. All Pontiac Grand Prix from 1999-2009 that leak water on passenger floor is from the following: The front cowl panel, [Where windshield wipers are] has a air box inside the panel, that when built has a two piece construction that is sandwiched together at factory, and over time, summer heat, Icy winters etc, dries out and allows water to get into air box and drip out below dash on passenger side. To fix: Remove wiper arms, remove cowl screen, and you will notice a box looking square on passenger side, clean out in and around box opening top. Must be clean and dry. Next use ether urethane windshield sealant, or silicone water proof sealer. Reseal all the way around box, [Where metal body meets air box.] Make sure to re seal all the way around air box and let dry overnight. You can buy ether sealants at your local automotive parts store.
Tuesday, April 21st, 2020 AT 10:08 AM