Other than the AC drain clogged,a leak in the windshield seal can do that too.Water can run down the windshield post and settle under the carpet.A very small leak will pass lots of water.This can be checked by temporarily covering the windshield molding with clear tape.In the body shop we drain the wet floor by working a small pinpoint punch through the carpet in the lowest spot then strike it through the metal floor.If left wet,the padding will soon mildew.