Is your screen 4", 8", or 12"? It's really that easy to tell.
Since you have the Premium(comes with an 8" screen) but no heated steering wheel then you don't have the Premium Plus that would come with the 12". An 8" screen which means no factory NAV in your car.
There's a good chance that it's the APIM but that's certainly not a wise decision to assume that it is without diagnosis especially at over $700. There's also apparently at least 2 optional APIM modules for cars with NAV: Sound System for 2018 Ford Mustang | TascaParts.com
You need your VIN to confirm the right module.