I'm in the process of installing an aftermarket amp/speakers in my car. The amp will live in the boot, next to the battery, so power/ground are nice and easy. However the amp will need a "remote turn on" wire to tell it to turn on when the car is on, and off when the car is off, to save the battery. Normally this would come from a wire on an aftermarket headunit, but as I'm keeping the stock headunit/idrive thing I need to find an alternative wire to use to tell the amp to turn on. Since the fuse box is next to the battery, it seemed to make sense to use something from there.

So, can anyone point me in the right direction for the best way to take an ingnition switched live off the fuse box in the boot of an E65?