Reverse camera LEDs turn on but camera won't come onto the screen

[the evil twin]

I'll give it a go thanks mate. No idea it's some chinese eBay unit which is performing quite well for $250 so far hahaha

Sent from my ONEPLUS A6000 using Tapatalk

On the pic you posted... Plug A is the plug in question and one of the wires will be labelled Rev Cam or similar.
If you put 12 Volts on that wire it will trigger the Head Unit to display the "R-Cam" video... that is what the red wire is for that is part of the RCA lead.