If this is old news disregard.
The engine shutting off is aggravating when you open the door.
This worked on my 14 GT automatic
1 Find the over ride button (taped to a wiring harness near the computer plug in port.)
2 Turn ignition on.
3 Press brake and hold button at same time for 10 seconds. Horn will blow 3 times. Release brake and button.
4 Press button till horn blows 4 times. Release.
5 Press button till horn blows 5 times. Release
6 Tap brake 7 times till horn blows 7 times
7. Press remote once.
8 Turn off ignition.
It worked for me. If it doesn't work for you go to UTUBE and Google Mustang remote start over ride and find guy with 2013 Mustang