Bought a Pioneer AVH-X7500BT to replace my single DIN stereo. Got it in, however back speakers aren't working? Tried an adaptor from halfords that allows all the 3 connectors from the car to be connected but still no luck? Don't understand what I could be missing tut any ideas? The stereo is this >> AVH-X7500BT Pioneer - Navigation, GPS - Car*Entertainment in case the helps. Thanks!