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.