We have a huge amount of water gettin into the rear passenger floor and some in the front floor when it rains.

 

do you have a sun/moon roof? are the drainage holes plugged up?

 

Had this issue with my 2006 Malibu w/sunroof...turned out that the drainage channels embedded within the roof that lead to the drainage holes were not built properly. Had to take to a body shop to fix and remove mold...just over $700.