I believe some people have just replaced the current speakers but had to do some extra work. Theres some threads on the Bronco6G forum where people have done some replacements. One thing to note if you do get the sound bar, it will pretty much cover all of your back window view in the rear view mirror.
I didn't look into them too much but here are some examples.
https://www.bronco6g.com/forum/threads/how-to-factory-bronco-speaker-replacement.68888/page-2#post-1853096
https://www.bronco6g.com/forum/threads/how-to-factory-bronco-speaker-replacement.68888/
https://www.bronco6g.com/forum/threads/sound-bar-worth-it.69205/
I didn't look into them too much but here are some examples.
https://www.bronco6g.com/forum/threads/how-to-factory-bronco-speaker-replacement.68888/page-2#post-1853096
https://www.bronco6g.com/forum/threads/how-to-factory-bronco-speaker-replacement.68888/
https://www.bronco6g.com/forum/threads/sound-bar-worth-it.69205/