I put a deck mounted sub in my E39 and love it. The additional amp was installed above the battery and the sub mounts directly to the rear deck lid. Clean install and super sound!
2011 550xi Carbon Black/Oyster, M-Sport, Premium Package, Technology Package, Electronic Damper Control, Dynamic Drive, Comfort Access, Active Seats, Surround Cameras
2005 X5 Sterling Grey/Truffle Brown, Premium and Cold Weather Package, DICE iPod Kit, Bluetooth, DSP, Comfort seats, Invision DVD Headrests, Valentine V1 hardwired, TomTom GPS