Check the label on the top of the RNS-E. If you have “C: Ext Control Connector: 2. RFSL” the screen will auto-switch to the camera view when the RNS-E is turned on and reverse gear is selected. (RFSL is German for Rückfahrsignalleitung or Reversing Signal Cable).

In addition, the RNS-E needs to be re-programmed to recognize a camera using a VAG-COM. Go into 37 NAVIGATION , 10 adaptation Drop down box , Select reverse cam, Change from 0 to 1,Save.

You can follow this link to see what's involved and what you need to do this mod. The camera listed for this project is a one-to-one replacement for the OEM license plate light but can be tricky to find so hunt around, they're out there -
https://www.ttforum.co.uk/forum/viewtopic.php?t=981945