Thanks for your patience.
The moisture entering your driver side passenger compartment is caused by a faulty seal on your AC drain tube. This seal, when failed, will allow water to enter two ways; when driving in wet weather, water from the ground will enter the vehicle, and the more common complaint is with the air conditioning on, the water runs out the tube and back into the interior of the vehicle.
Think of trying to pour water out of a glass, how some of the water will run down the side of the glass. This is exactly what happens with the AC drain tube; some water drips, and some runs back down the bottom of the tube. When the seal is failed, this fluid running down the bottom of the tube continues straight into the vehicle.
The bad news is, replacing this seal requires removal of the dashboard, crash bar, and full HVAC box. This is neither cheap nor easy, and as I'm sure you've seen the dashboards in these cars are very fragile and rarely survive this repair in one (or even 5) piece(s).
The good news is, there is an alternative repair we perform to fix this problem much easier, much faster, and with no risk of dash damage.
To perform this repair, you will need the following:
10mm socket, extension, ratchet
"dum-dum" puddy type sealant. My personal choice is 3M WindoWeld
You may be able to "borrow" some from a local glass/body repair shop to save some money. You will need about a foots length of the product; the smallest you will find in retail package at auto parts stores is typically 15 feet or so.
Start with your engine off and cold.
Remove the air intake tube connecting your air filter box to your throttle body. It is held on with hose clamps at either end, fixed with 10mm screws to loosen them. There is an additional small (~5/8") rubber hose attached to the middle. You can remove this, or just set the intake hose aside with it still connected.
Looking down the opening you just made, look just to the passenger side of the steering column shaft where it exits the firewall. You will see a black felt lining on the firewall that ends right in that area. Grab hold of it and pull it up, or tear off the extending corner (it is roughly 4").
With the felt liner removed/folded out of the way, you will be able to see the AC drain tube protruding through the firewall. It is roughly 1/2" diameter and extends 1/2" through the firewall. It will be extremely close to some hard pipes.
Take a few inches of windoweld or similar glass sealing puddy and roll it into a 2-3" long worm and gently place it as close as you can to the drain tube. You can not watch while you place it unless you are quite the contortionist, so just place it as close as you can then check your work.
Keep adjusting until you get it either directly under the tube, or down one side of the tube, but importantly making contact with the tube.
Once you are satisfied with the placement, use your index finger to press the sealant around the tube, and most importantly into the cut-out hole of the firewall. Keep pressing it in repeatedly until you are satisfied with the area, then repeat with another section over an area you have no covered yet.
Continue until you have a solid packing surrounding the entire drain tube; you want a nice smooth transition from the tube, to the sealant, to the firewall. Where once you have a hole with a tube going through it, you should now have what looks like a tiny black volcano on it's side. BE THOROUGH, and make sure none enters the hole in the end of the tube!
I know, it sounds hokey, but once you see what you are working with and where the tube is, it will start to make sense.
The alternative, the 'proper' repair again is to remove the dash, crash bar, HVAC box, and install a new foam seal. Unfortunately these seals don't last forever (typically 2 years at best from my experience), the risk of the dash cracking (worse) is very high, and you need an AC recovery machine to perform the repair.... making it a pro-only job with an estimated labor of ~$450.
If you opt to have a shop perform the sealant repair, expect to pay around $100. Someone familiar with Eclipses/Galants can do this in about 30 minutes, the labor is more to cover the ibuprofen afterward for the pulled muscles :)
If you have any questions, please let me know. I run into these once a week and have been meaning to take pictures for ever, unfortunately I have none at this time.
By the way, as a courtesy I checked your vehicle for open issues/recalls, and found you have an open recall for a fuel line that is prone to fail. Your local dealer can perform this inspection for you and if the incorrect/known bad parts is there, you will receive a new fuel line/pump module at no cost to you.
Edited by Doug C on 7/30/2010 at 10:10 PM EST