Searching the web for ideas on speaker installs for the E3 I found this install on an E9 (love the color... ) for rear seat divider subwoofer. Very discreet and I bet it pumps out pretty good sound.
http://www.benchmarkmotoringgallery.com/bmwcs_proj/bmwcs.html
http://www.benchmarkmotoringgallery.com/bmwcs_proj/bmwcs.html