Check all the other possible culprits first, because they're much easier to fix. I had a lot of water pooling in my 122 sedan's trunk also, which I unfortunately ignored and the result was a rusted-completely through seam along the inside of the truck, beside the fender. MY culprit was a leaky windshield seal, although there WAS NO water or moisture present along the package shelf. There was a tiny rust stain on the car's body, peeking from underneath the rear window aluminum trim. That should have been a clue, but I was too ignorant to pay attention to it at first. When I removed the rear trim to prepare for some bodywork--shazam! There it was: a quarter-sized rust hole, conveniently hidden underneath the bottom trim, inconveniently dumping water into my trunk on any rainy day. I think leaky window seals are fairly common culprits for interior water, particularly if the window rubber is old (a few cracks in the rubber, even "hairline" ones, are a good sign you probably need a new seal). But I let my car sit idle and forlorn for a long time, hence the rusty consequences....My car now has new seals on almost all windows, and is nice and dry :)

cheers,

Holly