I have a 2017 Mustang. I found online that on the top of both sides of my rear seats there is a loop. Just pull them and the seats fold down. Hope your model is the same. Good luck.

 

He has a convertible, can't fold the seats down on the convertible, unlike the coupe. The convertible back seat is one piece, and there is a metal bulkhead behind the seat.

Trixiexoxo -- call a lock smith. I had the same thing happen to me leaving a client meeting with my 2006 convertible. Pulled my suit coat off and tossed it in the trunk, and realized my keys were in the pocket when I closed it. A locksmith came out and picked the lock to pop the trunk. Then he made a key on the spot with no chip in it that I could keep in the center console in case it happened again. With no chip it wouldn't start the car and was only good to open the trunk.

 

Thanks for the locksmith advise.
I locked my keys in my 2011 convertible trunk : (

 

You can jump the wires under the dash that open the trunk.
It will take a little time but you can add a switch in the glove compartment at the same time.
This is one of my projects soon, sounds simple enough.