My father’s 2001 Buick Lesabre leaks when it rains, even when it is not moving. The water accumulates only in the floorboard behind the driver’s seat. This must be a common problem as there are hundreds of links on the web describing the same problem but GM has had no response at all.
So nobody at these links knows where the water comes in? My guess is the drainage system in the doors or a door seal.
What response would you expect GM to have? What is the problem? Door deals, blocked door drains or a sun roof clogged drain? These are common on all makes and models.
Have you tried to find the leak with a hose? Someone sits in the car while you wet an area outside. Are there any water stains on the headliner or inside door panels?