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
2013 X5 5.0 Space Grey/Black, M-Sport, Premium Package, Technology Package, Comfort Access, Surround Cameras