So i don't know if this is in the rite place, anyway my 2000 cavalier 4 door has water coming in from somewhere under the passenger seat under the dash.
does anyone have a clue why this might be happening?
heater core? just a guess but im know mechanic
If it's just water, try cleaning out your cowl really well. the drains can plug and cause water to spill into the heat and ac ducts.
first of all I would look at the where the water is leaking, See if the water feels like it has oily feeling. I would guess its youre heater core as well. You could allow youre car to idle, Then see if you can see any coolant or water leaking youre heater panel area.
Good luck.
I would guess it is your heater core also. smell it, does it smell like anti-freeze? if so its your heater core.
do you have an amplifier power wire running into your car in that vicinity? That was my problem, had to reseal it.

-Chris