This is either your heater core is leaking or your evaporator tube for your AC is plugged or is disconnected from the evaporator case. Is your radiator fluid going down - does your heater work? When your AC is on is there water dripping on the ground on the passenger side right at the fire wall? If you have water on the ground where the drain tube is then it is your heater core. I have put a couple of pics for you. The first pic is the inside passenger side of a car. You are looking at the hose I am talking about for the AC. This hose goes into the engine bay. Next pic shows the hose from the engine bay and a close up. See if water is leaking from here with the AC on. If it isn't it may be plugged. Let me know what you find.