Been there it's def. the seal on the bottom of the windshield it fell off and it still laying there under the vent for your windshield wipers re-silcone it change the in cabin air filter right there because it got wet and your done happens to impalas too as well as my 99century
It could be the heater core leaking.
The drain line for the cowl is stopped up or the windshield is leaking.
Antifreeze leaking from the heater core.
I have water leaking into a 1997 Buick LeSabre problem. I cannot find where it comes into the car. It pools in the rear passenger seat driver's side floor. It can happen even while the car is parked.
if it's not raining, the heater core is leaking
passenger floor
Passenger side floor? Could be leaking heater core
you could have a heater core leaking or your windshield is leaking and the water is running down behind the dash and onto the floor.
Heater core is leaking
There is a drain on the outside firewall that is clogged.
heater core may be leaking or hole in floor
Leaking heater core.