2001 Buick Lesabre floor board is wet after rain

Of course start by checking the door seals to be sure they are making contact all the way around.I have also seen them take on water at the base of the windshield on the passenger side where the heater+a/c take in fresh air. If the little drain gets clogged, the water will eventually go inside the heater box and leak out on the floor, enough water gets in and it runs to the rear floor.