Empty plug between passenger and rear passenger door

[Exploring XLT]

Just trying to figure out what this plug is/could be/could have been for

It is just below the front passenger seatbelt mount

 

[Roadrunner777]

That looks to me like connector C317. C317 is only used when the audio power amplifier is installed inside, over the RR wheel well. My guess is that they install a short harness from this connector to the power amp when they install the amp. If there is no amp option, this is just an empty connector. If installed, this connector carries the amp speaker outputs to the main wiring harness, and then on to the speakers themselves.

 

[massacre]

Haha I just went through this!
that connector is indeed for the factory amp harness, it goes from that location rearward under the back seat to the amp, and also forward to up under the dash and behind the radio.

 

[Exploring XLT]

Cool, I didn't test the voltages, but all of the prongs made my test light come on.

 

[massacre]

I just disconnected and removed the stock amp and head unit and left the harness in the truck.
using separate amp with line level outs and separate speaker wires.
thought about removing the harness but seemed too involved at this time.