Yea I got it now lol I would try the tailgate and see if it will set the alarm off with the doors disconnected and if not I would look into what triggers the alarm to be on. I would assume it's a wire that is in the door connector but I could be wrong.
Idk how Ford disables that when the doors are off but I would guess it's in the connector for the door? Maybe find a diagram and see what wire sends that signal and make a jumper or dummy connector to trick the system? Idk how that would work with the doors being off. I would have to see a...
Idk if Ford stamps the rear end with the information but you can jack up the rear end (do it safely). Mark the tire and driveshaft, rotate the tire a full revolution and count how many times the driveshaft makes a revolution.