What
@Deaky said...
The radio needs to see the speaker resistance to fully enable the rear speakers (eg 4ohms).
Some ppl use "high-level to low-level " Signal converters to feed the amp.
Some amps have high level input on board.
Then.you need to enable the 4 Chanel in vcds.
Then a radio reboot.(press and hold power button untill the booting logo displayed)
And possibly a reset/sleep for the settings to apply.
But as above it depends on how the rear speakers / amp have been connected.
Got any pics?