Rear speakers and sub woofer not working

[Guy Ramshaw]

I have just bought a 22 plate Outback Touring and still getting used to it. I have been using the radio and noticed that the rear speakers and sub woofer don’t work. I have gone into the settings and although music comes out of the front door speakers when I adjust the bias on the screen to the rear there is absolutely no sound. I have looked at the fuses and can’t see anything wrong but I noticed attached to the flap a “Sigma” keypad which I can’t find in the handbook. 
Has anybody got any suggestions?