Ok, this could happen to you. My wife has a 2007 Mustang Convertible. She locked the keys in the trunk. Bad for me because I have a bad habit of losing things ....like the spare key.
I didn't want to call a lock smith, AAA, etc.
I thought perhaps I could remove the rear seats and reach in the trunk: NOT. The convertible top tray (if that's what it is called) will block access.
Basically after a few mistakes, the EASY way is to locate the trunk release relay under the hood. It is right in the fuse box, and very accessible. Pull the relay and short pins 87 and 30 with a paper clip. It is actually called the "Luggage Compartment Relay".
I took me about 1.5 hours with removing the seat, incorrectly looking up a 2010 MY wiring diagram, disassembling things that were unnecessary to figure this out.
Now that I have learned the hard way, you can do this in 5 minutes even if you have no mechanical or electrical ability. After you are done, go have a beer. Of course tell everyone you figured this out on your own.