So I ended up changing the head unit to a Sony after being convinced by the audio guy that it would be a direct replacement without any modifications needed. Connected it all up no problem but no sound from rear speakers at all! Went back to the shop only to be told that on their system it doesn't list that my car has an additional amp & I would need to make sure that the amp wire was actually connected to the blue wire on the wiring loom that came with the stereo.... I found out that the amp wire is no 5 A & made sure that this was connected to the blue wire but still no sound! Any suggestions please as it's really frustrating..... Please note that I didn't add a sub, just standard speakers but new head unit.