Try earthing it out on the metal behind the rear seats, just pull one of the side top seats out, they just slide up then out if you didnt know. There will be few holes for you to choose from. This is the most common and cleanest earthing point people use in commodores. I'm not too sure about using the boot hinge....That way you will know for sure if it's a problem with the earth. You dont have any loose wire threads around the terminal sticking out or anything do you It only takes one little strand of wire and it can stop everything from working. What gauge wire are you using for the earth, if your not sure, then your earth wire, should be at least the same thickness, as your power wire. Remove the fuse from the amp aswell, while your doing all this...