I am planning to add an amp and a subwoofer in the trunk of my X5. I know exactly where to get the sub signal from. I know also how to power the amp from a constant +12v from the power point located in the trunk. The problem I have right now is to find another +12V working with the ignition key for the amp remote to turn on and off the amp. Any idea where I can get this +12v from somewhere in the trunk? All the +12v wires I found were constant +12v.