2001 buick regal floods when it rains (all windows up), is there a broken seal, gasket or plug broken somewhere?

Possible leaking through the cowl. Open hood and clean out area between fender and firewall. Also find heater box drain and run a wire or something up it to clean it out. Pop mesh screen off cowl and clean any accumulated debris out. If it proves difficult to find source of leak, pick up some dye tabs from any auto parts store, places these around perimeter or windows and run water over it. The tabs are different colors, so when a certain color shows up inside the car, you know which one was over the area that leaked.

do you get water in the trunk? more in front floor or rear floor? Possible problems, Rear window leaking at the bottom one or more doors have their drain holes plugged ...water that runs down the window goes into the door and the drain holes are plugged so the water run into the car via the door