2001 Mitsubishi Galant where is the rainwater drain plug?

You don't need a drain; you need to find the leak and repair it. Have a helper spray the windshield with a garden hose while you watch under the dash and around the window glass. The windshield is the most likely source of the leak but look at the fresh air duct and door seals too. If the floor only gets wet when you're driving through the rain, look for a front door that is misadjusted and the front edge is sticking out beyond the rear edge of the front fender and catching the wind.

Oops. I assumed it was on the front floor. For water in the rear, yes, suspect the sunroof. I only worked on one to replace the roof on a crashed Dodge truck. That one had four flexible drain tubes; one running down from each corner of the glass through each of the four pillars. I could see how a detached tube or debris could lead to water building up.

The diagnosis is the same. Sit inside while a helper waters the car. I don't know where those drain tubes exit the body but I suspect you're going to find water dripping right by the glass. If the glass can be opened, look for debris in the channel around it, and look for those drain holes. I doubt they're going to rely on just one, so look for at least one hole on each side.

You can forget about the doors sticking out on their front edges. Not that much water would come in that way, plus, you'd have to be moving for that to happen.

If water is dripping from the glass, there's two things to check. The first is to be sure the drain tubes aren't blocked, but in case they are, there's usually a rubber gasket around the assembly that seals it to the roof. In the case of the truck I helped with, that assembly can be unbolted, lifted up, and a bead of weatherstrip adhesive was used at the factory to seal it against those blocked drain tubes.

Also watch for a trickle of water sneaking in under the glass. If you stick a dollar bill under the glass when you close it, you should feel some resistance when you pull it out. Chrysler also has a white powder in a spray can for finding wind and water leaks. Spray it on the glass or the weatherstrip, close the window slowly, open it, then look for where the powder didn't transfer to the other side. If there's a gap between the gasket and glass it can usually be built up by putting a bead of RTV sealer or weatherstrip adhesive under the gasket, then leaving the glass slightly open until it sets up. For a badly deformed gasket, (those are usually hollow tubes), cut a slit across it near the deformation and slide in a piece of vacuum hose. That will hold the foam gasket in its proper shape but it will make it stiffer in that spot so it might allow a leak near either end of the hose. In that case you have to slide in a smaller diameter hose of something that is softer.