We experienced this. The air conditioning condensate drain line may back up and cause water to run into the passenger compartment. If you pull back the carpet on the passenger side up front, you can access the condensate line. Take the tubing apart and blow compressed air through it. It happened to us once, and we haven't had any problem since. I pulled up the carpet in several areas and on used a wet vac and towels to soak up as much moisture as I could. Then, I let the car sit in the sun with the windows open to fully dry. Fortunately, this tends to happen in summer when you are using A/C, so you can get it to dry readily. 2nd answer: The first response is good advice. As a suggestion, since the typical homewoner probably does nto have compressed air hoses laying around, is to to take a copper wire or soft-iron coat hanger wire and stick it up the tube. Have a small container around because when that tube is cleared, a whole lot more water may come gushing out. It is important to make sure your carpet gets completely dry or else you can be mold and mildrew and that is a real big problem to deal with.
Check the front passenger floor pan toward the fire wall.
Look under the passenger side floor pan where the fire wall begins.
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.
Knowing how a car functions is important. The "No gas" symbol on the passenger flood panel is the Fuel shut off switch.
Check the undercarriage for rust holes. My Bonneville would flood when it rained because of a small hole in the passenger side rear wheel well.
Was it coolant? Possibly a leaking heater core.
If it's the front passenger side ,I would say your heater core is leaking .
Windshield seal? Lower door seal? Hole in floor ?
It could be the heater core leaking.
Heater core has failed.
clogged or damaged catalytic converter
Heater core