It doesn't look like someone already installed the trailer wiring harness. Mine was just tossed in the back when I got mine new from the dealership. I ended up installing it (took about 10 minutes) and then hooked up the trailer. That was when i found out that the vehicle needed to be flashed for it to work. I ended up getting it done for free since I convinced the dealership they should have done it in the first place.
Bottom line, if you don't see a 4 pin connector back there already, you don't have the trailer wiring harness installed. Luckily it is a plug and play system and very easy to install. Good luck!