My Bonneville had water on the floor in one foot well. Searching for and using a GM Technical Service Bulletin, helped solve a baffling (no pun intended) mystery.
I used my new information to take a close look at the bottom edge of the trim panel right after a good rain. Sure enough, water was dribbling from between the vinyl trim panel and the metal door shell, and onto the floor.
I resealed the plastic water barrier panel (immediately behind the door trim panel) with butyl ribbon as specified (I could see voids in the factory applied butyl).
Voila! No more water on the floor.
CSA