If what you say is right and your friends ride is a 316, it should also mean that it is either an E30 or E36. Either way, the stereo is not integrated with the electronics in the car, as such, it is very easy to mod your ICE. however, please note that the headunits that came with the cars are utter and complete crap. As such, it would not make much difference if you add a bass tube or subwoofer. Better off going with a newer and better headunit...