I think there will be two parts to finish that off,

1. When the dealers fitted those kits, there was a jumper to connect the can bus to the rear (can signals don't like disconnected ends) 5N0055204N, here is a guide to make one up https://www.vwvortex.com/posts/115860071/ or you can splice the lines, this connects at the BCM, right above the accelerator pedal you will find a connector. you may have done this already (The brake lights go direct for safety, so that might explain why those work)

2. Coding, Trailer Hitch Retrofitting (3C) - Ross-Tech Wiki
The main parts are,
- Register 69 - Trailer in the gateway
- parking sensors and RVC coding updated.
Don't worry about the Trailer Stabilization coding, which was not approved for use in Aus.