2000 Nissan Altima Water leaking on front passenger floor

On the firewall in the engine compartment, find the duct work that houses the heater core and AC evaporator. (It's the big plastic housing)

look for a little tube, or sometimes a short hose, that doesn't attach to anything. It's just hanging there, looking out of place, and probably pointing down.
That's the drain, get a stick or something (i use a metal coat hanger) and try to remove the debris from the hole.
Try not to simply push it back up in the hole, cause it will eventually find it's way back and clog up again.
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