Why does a 2003 Hyundai Elantra have clear water intermittently leaking under the dash on the passenger side and if the vent drain is clogged how do you clear it?

I had this same problem with a 2003 Hyundai Elantra. Mine turned out to be the AC drain. It is located just a little left of center line near the firewall. If you get under the passenger side near the door you will see the little black stub near the exhaust pipe in the center. http://www.hyundai-forums.com/index.php?showtopic=16157&hl=drain I once had a problem just like this with a 1991 Pontiac Grand Am. In my case I had to clear all of the leaves and junk that collected in and under the windshield wiper trench. When I poped the hood, there was a slight gap on each side where leaves would collect. With the hood open, the gaps were in the top corners. In my case, the leaves would become brittle and break up into small pieces which would fall down and clog the water drain channels. As it decomposed it bace a thich mud that would not clear on its own. When it rained, water would not be able to drain and would instead build up and flow through the air vents into the car interior. Look for openings in your car's engine and frame that might allow leaves to collect. Clean this out as best as you can. For me, I removed the wipers and lifted the cover just underneath them and did a good cleaning. I then also repaired any cracks in the cover that cuold let more water in than was necessary. I hope that will give you some benefit. One I idea I also had (Which I am not convinced Is a very good idea because of complications) was to drill drain holes into the car floor to allow any water that did collect drain away. But you have to make sure it doesn't start a rust problem or have you damage anthing underneath the flooring. Good luck.