It is probably the heater coil. When it stops working water from the radiator goes back through a hose and when it overfills it emptys into the passenger side floor. You can replace the heater coil or bypass the hoses. Second option will cause heater to stop working.
This you may determine for yourself, by measuring the area of floor mats; both front and back; in your car, or in a neighbour's car.
1st bet: Does it smell like antifreeze? Leaky heater core. 2nd bet: Worse after rain or car wash? Leak under cowl vents (the vents in front of the windshield. Good luck!
The source of the water is probably from faulty or failed window seals surrounding your front & rear windsheilds.
I hav e the same problem
pull the E brake all the way out and floor it!
Heater Core is the likely culprit
it is a height requirement now. pretty much if they can sit in the vehicles seat and touch the floor with their foot flat on the floor they are safe
You could place them in a box on the floor in the front seat and turn the heater on medium setting. You could wrap a hot water bottle in a towel and place that in the box with the ducks.
== == There are any number of ways you can get water in the car after a rain. You did not say the location of the water. Driver side front, pass. side front, rear cargo area, in the storeage bin area? With out knowing where your water is, it will be impossible to give you a confirmed answer. The water could be running under the top and then coming in under the trim and finally onto the floor. Water could be leaking from around the windshield and hidding in the moulding and kick panels finally on the floor. The rear hatch window has no drain holes and the weather seal is the only thing stopping the water. If the glue has seperated and there is a gap, once again, the water can sneak in under trim and flow down to the floor and storage areas unseen. Take your car to a good body shop and they should be able to find the leak. Be warned, it may be costly.
You didn't mention if it was a car or truck and what series. About the only 2 possibilities are the heater core or windshield.
you will find ECM center front (inside car ) on the floor hide by black plastic.
There is a conditioning system drain hose. Eventually it starts to leak inside of the car. Fix the hose.