This has been known for a while. It does suck, allegedly because it was too hard to get them. People were give $250 off their total I believe, after initially ordering it. Still a pain for someone who is not so inclined to add it later. If they cant do it themselves, then will need to pay someone. So the cost will be more than the $250. No amount of bitching will change that they will not install the sub for you on build, which really sucks.
For me, I didn't get to pick my options, so I didnt have the higher end audio. I changed all 6 speakers, and even changed the back 4";s to 6.5"'s. Then added a 12" sub in the back, and two amps. One for the sub, and one for the other 6. The sound is so terrible, changes must be made. Even just swapping the 6 speakers out, made a huge difference. Then adding an amp to the head unit, made it actually enjoyable. But I wanted some thump.
For me personally, the sound system is the single worst part of a Bronco. They are meant to be drive in the open air, top off, doors off, etc. They normally have very large tires, that make more noise. The stock system is simply TERRIBLE. Cant enjoy music when driving in the open air. Tin can sounding, no base, and quiet. So upgrading was a must for me. The Stinger box came with a Phoenix Gold sub, swapped that out for a Rockford Fosgate. MUCH better sub, and the Phoenix Gold came with a crushed screen on it. I have security cage on the back to put her purse, ipads, etc when we have top off. Had to trim it for the sub box to fit.
Few pics of the sub and setup.