I bridge Code and +UB (code may be named differently so it is the one above +UB) when powering up externally, but I do not suggest bridging any of your car wiring as it is already in the loom!
It does only have one power supply in the loom so to speak, the ignition switching is done via the CANbus hence why an OEM or direct for VAG HU has this protocol integrated.
You should only need an external supply if you are fitting any old generic stereo as this doesn't talk to the CANbus.
Picked up a 12v supply from the fuse box by the OBD plug, seems top row are battery (perm 12v) supply bottom row are switched (ignition) I fitted it into the "missing" wiring hole in the quad lock, the head unit now works fine, switches on and off with the key and all seems well. now all that's left is a DAB/FM/AM roof aerial and the aerial splitter on the back of the RNS 315. Over to you, who's done and and what are the part numbers please.