There is a plastic housing for the A/C evaporator on the passenger side.
When the A/C is running, water will form inside the housing from
condensation of moisture in the air. At the bottom, front edge of the
housing, there is a nipple where a drain hose is connected. The hose
goes almost straight forward through the fire wall and then bends down
inside the engine compartment. Check around this hose, perhaps it has
been disconnected from the nipple, perhaps it is broken/ torn, or
perhaps the hose is clogged so the housing fills with water and
overflows.

Agree with Randolf... your drain is plugged.

Professor
www.telstar-electronics.com