Just some alternative notes: I have both, a real GT 350, early car with similar panel, and I'm building a Tribute car with this same similar panel.
1) might be the 6/32 threaded studs/nuts that hold the aluminum trim has been over tightened. But, in order to fix that, you might have to unbolt it, clean out the sealer, which is 3M strip caulk, and then be careful when you retighten the trim. Just a little cualk needs to mush out, so do this when the temperature is above 60 degrees so the caulk is soft.
2) on my tribute car, I also did not like how loose the fiberglass panel was across the top. This is more work but it fixed the problem. I split the length 3 ways, and in the 1/3 position, and the 2/3 position (above each taillight). I fiberglassed in a one inch length of steel tube. I believe it was 11/32 ID. This tube sits in the depression along the tp of the taillight panel. Then I bent two pieces of threaded rod (again, 11/32 thread) so they could slip into these tubes. Drill a small hole in the appropriate spots along the top (but below the area you would see) and reassemble. You now have two more hidden fasteners to hold that area of the panel. Except if you look from the inside and up, you can see the nuts.
Does it matter on a tribute? Very little. If you are careful, you might not even mess up the paint on the tail panel.