2004 Mustang GT Premium

Could be for any reason.
My guesses are someone went all out with subs and speaker boxes or the trunk seal,specifically on the driver side where the wire harness goes in over time wears the seal down and causes heavy rains to get the inside of the trunk wet and enough will ruin the carpet and whats basically cardboard.
Happened on my old 01 GT and this past XMAS happened on my Mach. I know of some other stangs in the area that had the same issue.
I had to rearrange the harness then add low heat to the trunk seal to get its shape back so it would seal.

OR maybe someone was going for weight reduction...