Found this on JustAnswer.com...sounds pretty legit, was credited to a Mitsu technician. Anyone tried this? Sounds like threading a garden hose through the eye of a needle to me...
This drain is only accessible from underneath the vehicle. Note that if you have an AWD, you really need to have the vehicle on a proper lift to perform this repair. It is simply to tight to do lying on the ground
Access from the top is 100% impossible, so for a FWD you will need to put the front of the vehicle on jack stands or service ramps.
Laying underneath the vehicle, look at the center of the firewall where you will observe a set of metal brake lines running up the firewall then making a 90 degree turn toward the drivers side.
Right in the corner of that 90 degree turn you should see a hole drilled in the firewall of approximately 3/4" diameter.
You can pull the brake lines gently off the firewall (they are snapped into plastic looms) and stick your finger in the firewall hole and feel around. Looking straight at the front of the vehicle, the drain tube is inside the hole, down 3/4" and to the right (driver side) 3/4".
Needless to say, it is not extremely accessible. It is possible to get in there with a pipe cleaner or a compressed air gun with a 2-3" long nose on the end with practice. The first time will be a bear though.
Things to note:
If you only have water on the driver side and not the passenger side, and as well do not have any sloshing noises etc while turning, your drain is NOT clogged, but rather the draintube seal on the firewall has failed.
When this failure of the foam seal between the drain tube and the firewall happens, water runs down the bottom of the tube and runs right past the seal back into the car (think about pouring water out of a glass, how it runs down the side), thus only wetting the driver side.
There are two solutions to this problem; You can remove the entire dash and AC box and replace the seal (not fun, and around $700 in labor), or you can install an auxiliary drain hose onto the tube.
The second is not an official repair however the problem is so great on these vehicles Mitsubishi is doing this regularly rather than paying for dash removals.
The hose used is for the coolant overflow jug on a 00-05 eclipse. Is is exactly the same diameter as the drain tube.
To install, you will need to fish a flexible metal rod into the drain tube to guide the rubber tube, then slide the tube over it until it seats on the drain tube. This will permanently fix driver side floor problems.
Performing this repair out of warranty is typically about $150.