If you got a Marti report, it should show the option of "Visibility Group" on you car. Also, the fact that you have a plug on the drivers fender is a decent sign your car had that option, because I am pretty sure the rear taillight harness was different for those cars, and my cars don't have the plug you are talking about.
From the Mustang Recognition Guide, the Visibility Group consists of...
LH remote chrome mirror (n/a Mach 1)
Glove box lock
Lights in : trunk, glove box & ash tray
Parking brake warning light
underdash courtesy light (hardtop only)
lighted ignition switch