2001 Mitsubishi Galant leaks water into passenger side floor when Air conditioner is on. Whats wrong with the car?

The A/C evaporator drain tube is most likely blocked. If you have a V6, you're in for a real treat trying to reach the thing to clear it. The tube is on the firewall behind the driver's side rear motor mount, and slightly to the left as you're facing the firewall. It is a 1/2" diameter metal tube projecting about 1/2" from the firewall, and is covered by the firewall sound and heat blocking material which has to be folded back to reach the tube. A pair of what I believe are brake lines are one finger's-width in front of it, making it VERY difficult to get to the tube. (Brilliant design, Mitsubishi!) If you are very dexterous, you can slip a small piece of wire or a cable tie under the brake lines and into the tube from the front to try and clear it. Good Luck!