This isn't a question, it's a story. This is a problem I have already fixed with a trip to a mechanic. But I think some of you might be interested in this just for information.
I have a Civic EX '04 with about 50K on it, runs fine, I change oil, etc. I am usually the only one in the car when driving.

Last month I started hearing a sloshing sound sometimes when I would make a left turn. It was a sound that you might hear when briefly shaking a jug of water, and I thought that's what it was, as I keep a few water bottles in the car. I thought maybe I was turning faster, and that's why I never heard it before.

Weeks after that I had someone in the front passenger seat, and when he got out I noticed the floor mat was wet. On closer inspection I see the mat is soaked. I lifted up the mat and I see that the felt under it is soaked as well. And at this point I noticed a single drip from under the glove compartment. I suspected there was a leak.

Because the felt was soaked (just TOTALLY soaked) all on the passenger side, I took it out as carefully as I could after removing the front passenger seat. This took some time. At one point I ripped it in the back (behind the armrest) because it is apparently one continuous piece that connects to the driver's side felt. And of course, after removing it a pool of water was there, which I absorbed with a lot of towel wringing.

Then I decided to take the car out and make a lot of left turns. I did this in an empty lot so that I could watch below the dash on the passenger side. Sure enough it's slowly dripping water.

A trip to the mechanic (since I had absolutely no idea how to solve this) and he flushed the air conditioner exhaust which was apparently clogged. I live in an area with a lot of trees and tree debris, and what I suspect happened is that this debris somehow entered the airconditioning system (probably through hood) and made its way to that exhaust valve, clogging it. The valve is under the car somewhere near the center and it expels water from the system. The buildup happened very slowly, but over time gallons of water were accumulated and backed up the system. Yes, gallons. He put pressure to flush the system and gallons came out.

The backup caused a seal in the airconditioning dehumidifier system to eventually give out and then the water slowly dripped into the car.

Lessons learned: check that felt every so often to see if it's wet. And don't ignore any sloshing sounds.

If you guys can think of anything else I can do as a preventative measure, I'd love to hear it!!

 

It's the "condensate drain tube". Your vintage vehicle should have cabin filters in the air ducting that should collect most of any debris ingested through the cowl. Check/replace yours. They may be rotted out or mouse chewed. Typically, you drop the glove box by removing the retaining clips on the left and right sides of the box. This exposes a plastic flap you can easily remove to expose the the filters. It's simple and doesn't require any tools. Search or consult your owner's manual for a more detailed description of the replacement process.

 

Also check your sunroof drain tubes those like to get clogged too..