In front of the firewall, on the ouside of the car, under the cowl, there is a piece of weatherstripping that falls off. To find it, open your hood, make your wipers stay up on the winshield and take off the clips holding the black plastic piece. Make sure your cabin filter is properly in place or replace it if its dirty like mine was, then that weatherstripping piece is on the black cowl piece that you removed, make sure its there and if it is reglue it back on to be sure it seals well.
Are you running the air conditioner? If the drain line is plugged, it will leak water (condensation) onto the passenger side floor.
Probably have a leak in the heater core>
There is obviously a leak in either the firewall/cowl area or the window seal.
Maybe a bad heater core
because your heater core has a leak or the hose is broken
I have a 2001 impala and for me it comes on when I am too close to the airbag. If you just back up a little it should turn off. Being too close to an airbag could hurt you more then the accident you're in when it goes off. Check for moisture under the passenger side floor carpeting. There is a known issue with rain water leaking in from the passenger air vent under the front windshield. An air bag device is located on the floor under the passenger seat. This has most likely become corroded due to moisture. If this is the case you'll want to get the leak fixed before you bother working on the air bag issue. (do a Web search for "impala passenger leak" etc. - you'll see lots of complaints of this.
It could be the heater core leaking.
when your heater core is shot it will leak on the passenger side of vehicle and your windshield will fog up most of the time
Wet passenger flood is caused by either a leak in the air conditioning drain line or a leak in the windshield sealant or a coolant leak coming from the heater core.
If the leak is by the passenger floor-- could be the heater core check to see if this leak is engine coolant or just water. Could be a leaking windshield seal could be a door seal could be evaporator line is plugged
There is a conditioning system drain hose. Eventually it starts to leak inside of the car. Fix the hose.
Dry up the water. Take the car to a car wash and spray it good around doors and windows. If a leak is not visible then check the floor during rain storms while driving to see if water is coming in through a leak in the floor.