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I recently purchased my second F body, ironically enough it is the exact same color Z28 as my first. However after getting this one home, I quickly realized there was water everywhere. I tore the interior out and did some research much of which was here. (My first one was in pieces most of my ownership in a garage, so it never saw much rain.)
I’m sure I missed a lot of posts on this topic, but I haven’t seen anyone mention putting a drain plug into the trough/gutter where the water likes to collect? I was debating drilling my own hole there, sealing it with some 3M, and running an exit tube down to the already existing drain plug at each corner of the trunk.
You may have already found many or most of these threads, but here are some important ones that I've saved links to over the years (regarding leaks; ways to find the source, various fixes) - they also contain additional links within that might be of further help:
There are actually two rubber drain plugs underneath the trunk where the t-tops are stored. I removed one in a previous car to let a water leak drain out. I never did fix the leak, but at least I didn't have a pool of water. The plugs are underneath the car by the muffler.
Yep, both of mine are out right there from the previous owner. Mine like many others collect water at the rear corners and it ends up running down into the buckets for the rear seats (those plugs are also removed), and then up into the front pans as well.
Look at the FAQ Sticky in this section. Extensive links to GM's water leak diagnostics are posted there. You should be able to find the source that your PO didn't feel the need to address.
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Trunk Leak 
