CORSA LIFE _ FADER OPTION ? ? ? ? ? HELP!!?

Hi, does anyone know how to find the Fader in the new corsa life 5door 1.0L
? to allow rear speakers to work ?
Manual say 3 times that using the SOUND button should give options of Fader, Balance, Bass, and Treble - I only have the Balance, Bass, and Treble Options ?
I have had car a week now and am pulling ma hair out
Any Advice Out There ? ? ? :o(