I have an alternate possible answer to the person who had water accumulating in his trunk. I had the exact same problem with my 1997 Buick LeSabre about 4 years ago. Up to a couple of inches of water would accumulate in my spare tire well, and I could find no evidence of any leak around either the back window or the trunk seal. I finally found the answer one day when I began examining the underside of the car: I had a hole rusted in the top of the right rear wheelwell! When the road was wet, the tire would spray water onto the wheel well and the padding over the hole would then absorb it. It would then run down into the low spot - the spare tire well. I patched the hole with some bondo, and have never had another problem since! I never figured out why the hole formed in the first place, but can only conclude a defect in the metal.
I haad a similar problem, No visible water leaks from above, but 3-4 inches of water would appear in the spare tire well. It turns out that some minor body work had been done on the right rear quarter panel after a slight fender bender that happened before I bought the car. Well, they had fixed the visible damage but didn’t touch the inner piece of sheet metal. The inner piece had been bent where the plastic exit air vent is and when driving through water, water would splash up to the inner panel, run down and enter the trunk throuth the space between the bent panel and the top of the plastic vent. I straightened the panel and I have had no trunk moisture since then (over 1 year).
I shold have told you that my vehicle is a 2007 Saab 9-3.
I had the same thing on my G6 shortly after I bought it. Ran a hose a number of times to find it, pulled the tail lights and used seam sealer, etc. Finally noticed a rubber grommet that had come loose when I had a hitch put on. There are any number of ways water can leak into the trunk. Just have to get the hose out and start looking.
Another alternative. I had a 1995 Dodge Stratus where it was a bad weld behing the tail light. I needed to remove the light and caulk the weld.
If only Dodge had designed the car so that the top piece of metal overlapped the botton sheet on the outside, like house siding, it wouldn’t have been an issue. But there were a lot of design flaws on that car.
I was completely unaware that water was accumulating in the spare tire well of my 2002 Chevy Prizm 4-door sedan (metallic blue-green) until a frigid day one February morning when I found that a tire had gone flat overnight. When we lifted the liner to get the spare we found that it was frozen in place due to a couple of inches of water that had accumulated and frozen solid in the well! The car had to be towed so I could get the tire repaired. I later took it to a body shop, where the mechanic showed me that I had drainage holes with plugs under the spare, and he drained the water out. We replaced the plugs and he suggested just keeping an eye on it before trying to diagnose where the water was coming from (he didn’t charge me anything). That was years ago. I check it regularly - it has never reoccurred, and I have no idea what caused the problem in the first place.
Same problem on my 1988 Accord hatchback.
Turned out the gasket around a tail light was leaking.