I have a 2000 LeSabre - This summer (while hot and using air conditioning), I had water coming into my passenger side floor area via hear/air duct work. I learned that the rubber "beak" on the end of the air conditioning drain pipe, where the condensate runs out had melted and the opening closed up, and the condensate was backing up into the system. A friend removed the rubber part and water ran out onto the ground freely...no inside leak since. According to him, not necessary to replace the "beak", just left the drain pipe open, he knew immediately what to do, said he has seen it often. I do not know if you are located where you have been using air conditioning this time of year...but thought this might help.
I have also have had a leak around the passenger door seal for several years, when it rains REALLY hard, water comes in on floor mat. Dealership told me they have a "door seal" guy who comes around once a month and does that type of work, for initial fee of $75 just to diagnose, then more for whatever repairs or replacement. I never went.