The usual problem is there's a build up of dirt and other krud in the air
channel in front of the windscreen that's blocking the drain hole, so water
builds up. Fix it by ...

1. Remove the wiper arms
2. Remove the screws / clips holding the air channel cover intake grill.
3. Remove the grill
4. Empty out or suck out the water sloshing around in the channel. At the
lowest point, there's a hole that drains through a rubber hose or other
channel, down to the road.
5. Straighten a coat hanger and CAREFULLY poke down the hole. The bottom of
the hose is usually moulded as a pinch closure, and after a while the rubber
sticks together.

6. Dry out the channel area and make sure there's no rust developing. If
there is, treat it now.

7. Put it all back together.

I also have this same problem.  I found a post on this same group with
some additional information, but not a how-to on fixing it.

http://groups.google.com/group/alt.autos.mitsubishi/browse_thread/thread/967b8b8
0e6c8f773/e601f4674925221a%23e601f4674925221a

How do I unclog the AC overflow???  I guess that's what it is.  It
seems like when I make a turn, I have water rushing into the passenger
side floor and it sounds like its 50 gallons worth!!  It really makes
some water noise.  I need to empty it out but I do not know where the
hose or valve is.
Current Ride: 2002 GSXR1000 Red/Black/Silver/White
Location:  Northern New Jersey
Commute to NYC everyday
Previous Rides: 1988 Honda Rebel 450 (Sold)
               1991 Suzuki Katana 750 (Sold)
               1994 Honda CBR 900RR (Stolen)
               1998 Suzuki Katana 750 (Sold)
               2001 Suzuki GSXR1000 (Stolen)