So I bought a 2015 A3 the other day (love it, it's pretty much the perfect car). But there's a door on the right side of the trunk with what looks to be some sort of rectangular bracket behind it. What's supposed to go there?

I've asked like 3 or 4 salesmen during my shopping and none of them seem to know.

 

That will be somewhere in the owner's manual. Mine doesn't have that feature because it is a Sportback,but everything is covered in the manual.