This is in reference to 2021 RAM trucks upgrading to the UAX.
Confirm the RED Fakra is connected to the RED antenna. RED is for Digital Camera, which the New Body RAMs use for Rear Camera input.
There have been several instances of the 2021 RAMs that do not show any reaction to shifting to reverse (Even with the RED Antenna connected), as in the display will stay as it does in Park/Neutral/Drive.
This is a result of something to do with part number compatibility.
Replacing the UAX module with a part number manufactured for 2021 will fix this issue.