Check to see if the drains from the sunroof are clogged. If they are rain water will pool and run into the car.
Was it coolant? Possibly a leaking heater core.
Driver abuse.
Could be the heater core leaking or the windshield needs sealant.
Bad seal around the bottom of the wind shield maybe?
Consult a Local Transmission Shop
a lot of things mostly a leak of some kind
Most likely cause would be the floor has a moisture problem or the floor was not level when it was installed. There is always a chance if it is fairly new that it is defective.
Possible BLOWN HEAD GASKET!!! BAD NEWS
No
a bod driver
Leak on drain line above floor level
insanity