If no one with a 21 pipes in.... On my 2014 5.0TA I had the prewire for the stereo done too. It amounted to one end of the speaker wires in the upper cabinet just forward of the door, one set of the other end went above the dinette and the other towards the bed in front of the trailer. If I recall correctly, ETI was installing the head just under the cabinet where they ran the speaker wires. There is also 12v available in the cabinet. They were using one of the plastic under cabinet housings like this one
Just a WAG, but it may be they hung them the same under the cabinet you found the wiring in in your trailer.
I've yet to do anything with mine, so far the phone and a bluetooth speaker have been enough.