I have water under the passanger side seat that goes back to the passanger side back seat. What is this from?

Well if it leaks after a heavy rain, then suspect your windshield seal is simply bad and needs resealed.if it is anytime, then it sounds like you have either a heater core leak or a hose to the core.All that is contained under the dash on the passenger side.You will need to take off the plastic covers under the dash on that side then get up under the dash with a flashlight and look while the car is running.Let it get hot and turn the heat on also then just look for the leak.if it is really wet you wont have to wait long with the car running and the heat one to see the leak.