This is a simple sub & amp installation on a 2008 BMW 335.  It’s actually surprisingly easy to do a sub & amp install on this car.  The car come equipped with Logic 7 sound system and the stock amplifier just happened to be conveniently located at the left corner of trunk area.  All I need to do is to find out which wire is for subwoofer underneath the front passenger chair and use that signal to feed to the Alpine 12″ sub that he has.  This car is very much like others that have factory subwoofers, there are already plenty of upper bass.    So we adjusted the sub to have low pass frequency of only 50Hz and filling the end where the stock woofer isn’t capable of.