Water comes in on passenger side front seat under the seat and next to center console. What can be done to stop leak and detect where the water is coming from

Had the car 2 months and it starts flooding. There would be between 4 and 6 inches of water on the passenger floor board when it rains. It wouldn't be right away but after driving awhile. I took it back to the dealership and had them look at and they found that the water was getting into the A pillar but none of us couldn't figure out why or how.