So, I looked into this. It's not more complicated. The ground wire shorts out any signal that WOULD BE sent to the monitor. So you have to have the wire disconnected (or unpinned) AND have a source going into the A2 point. If that source was powered by the reverse light, then it would show while in reverse. If that source were powered directly from the battery, for example, then it would override and always show whatever the source is.
Also, if the ground wire remains attached and you hook up a camera, then it will NEVER show.
In short, you can't just unpin the ground wire and expect something different to show. (at least from my vehicle, which is a '15 Ultimate that came stock with factory NAV
If you already have a mirror setup for a reverse camera, then I see why you'd opt to just stick with what you have.