Here is another photo. Besides having a power lead that comes from the engine compartment, it has one wire going to this in-line splice, gizmo hooked into each tail light. I thought maybe it had something to do with sequential tail lights, but when they are unplugged, I still have sequential tail lights. Isn’t that a factory thing anyway?
One dealer add-on often involves specialized brake lights, a rapidly-flashing light. But the examples I've seen, they involve the roof brake light only. And the wiring is not near as involved.
They might have tapped the under hood fuse panel for a device needing power. I can see that. But why that would also involve the tail lights, IDK. With the under hood connection providing power, maybe they tied the device/s into the tail lamps, using them as a type of trigger (or as specialized lighting).
Edit,
Yes, I never considered that. I mean, how many people tow anything with a Mustang? Very few. Maybe not a full trailer, but something big enough to carry motorcycles, jet skis, or a small, homemade trailer.
The gizmo in your first picture, is there a wire going directly to your battery? If there is then I know what that thing is. The computer in todays cars are so sensitive they will throw a code if they detect an overload on the light circuit, like what happens when you plug in a trailer. So what you do is install a module that plugs into your tail lights and those monitor what the lights are doing and then the module duplicates the same function for the trailer, but all the power comes directly from the cars battery, not the tail light. I have one on my SUV.