most likely cause is your heater core leaking
Check for a sickly sweet smell. The heater core is probably leaking coolant into the floor. The core is not repairable but can be replaced.
Antifreeze leaking from the heater core.
The heater core is probably leaking engine coolant. Replacement is necessary.
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
Heater core leaking?
Leaking heater core.
Are you running the air conditioner? If the drain line is plugged, it will leak water (condensation) onto the passenger side floor.
If your talking about coolant leaking on the passenger floor then it means your heater core is leaking and needs to be replaced.