Has the function ever worked?
If not, the Pioneer tech support folks may just walk you through the procedure in your manual. If you haven't yet talked to them, the reference in the manual follows and my interpretation of what may (or may not) be your solution.
The rear view camera setting could be switched to "off" as that is the default setting. The "off" position works when you shift into reverse and the monitor switches to the "rear view" image. From what I'm reading or interpreting from the operating manual, f you want to override that, the rear view setting has to be turned to "on"
But I don't know, I'm not too intelligent so I may be not interpreting that correctly. Maybe you or somebody on the forum can explain what it's seems to be indicating.
It's on page 125 of your "operation manual" ...in case you've misplaced your hard copy, here is the digital version --to download,
go HERE: