My best guess is that the rattle is coming from the rear package shelf...area where the third brake light is.
Best way to get help is to find another person willing to lie in the back with the rear seat back down... Listening for the sound, then putting hands on where sound seems to be emanating from.
Dealership will put their least experienced person on the job...someone they don't pay much. This sometimes gets the problem figured out, often though not.
Rattles in package shelf area were very common on 2013 early production models. I don't know just what all the fixes were.
I have a 2015 FRS, no problem until recently, at over 50k miles on my car. My car rattles from package shelf area at 70 mph and then only sometimes. Wiggling the plastic cover for third brake light sometimes makes the sound go away yet not a permanent fix.
Best to have a friend who is good at being patient, working with you to find a fix.
Some package shelf rattle fixes within this Google search:
https://www.google.com/search?q=ft86...hrome&ie=UTF-8