Cabrio leak - need help

[dgjks6]

99 c4 cabrio. After 1 rainstorm 2 inches of water on the floor. Clean up, replace some seals, and clean drains.

2 of the rubber grommets under the hood were not even pushed through the holes. Did all of this and then drove tonight thought the great rain of the Midwest 2013.

Got home and carpets were damp. Drivers side only - front and rear. Disassembled everything I could and dried the carpets. Still could not find leak so I removed the rear interior panel. Actually followed directions and did not break anything (I don't think I did)

Under the rear passenger seat was also soaked. I found this:

I know I could have found it with just the speaker removed, but this was I learned something and I found under the seat (not the floor) was soaked

When I lifted the side panel and carpet I found this behind the side panel

and this fell to the floor

any ideas?

[dgjks6]

Ok. Took of the other seat belt and found another little metal ring. So got that figured out.

But still trying to figure out what the black plastic piece is.