It looks like it is for a cover that rolls up when your not using it. It was designed to cover things in the back area so nobody could see. You would grab it at the seat area and pull it towards the tail light area. You should see some kind of brackets by the sides close to the tail light area on the inside.
2006 Mustang GT Premium RedFire. CAI, SLP Loudmouth, SCT Progammer, 5 speed manual trans. My other car is a CORVETTE!