If you have a 2010 328 and you don't have tweeters, you have a truly crappy sound system, and a sub is the least of your worries.
This page explains which system you have: http://www.musicarnw.com/bmw-factory-audio-explained/
You don't mention if it's a coupe or a sedan.
Sedan with Base Stereo: http://www.musicarnw.com/e90-3-serie...n-with-stereo/
Coupe with base Stereo: http://www.musicarnw.com/musicar-sys...s-with-stereo/
IF your 2010 has Premium Sound/harman/kardon:
Coupe subwoofer: http://www.musicarnw.com/e92-3-series-coupe-subwoofer/
Sedan subwoofer: http://www.musicarnw.com/e90-3-series-sedan/