Front to rear fader

[tanoga]

I have an issue with the sound system in my R8. There is no sound coming from (what looks like) the rear speakers. When I adjust the fader to front bias the sound comes from the front speakers. When I adjust the fader from the front bias the front speakers progressively get loader as I move the fader to the centre position and then as I move the fader from the centre position to the rear bias the front speakers get progressively quieter but still no sound from the rear.

Anyone got any thoughts or suggestions as to what I am doing wrong or do I have some sort of a fault