What kind of car? Z4? Does the head unit have an internal amp with speaker-level outputs? Stock BMW amps are designed to accept either 'balanced differential outputs' which are closer in nature to a speaker-level outputs, OR a fibre optic connection from the headunit.
With E90 cars for example, any of the premium sound systems (H/K or Logic 7) use the fibre connection, so there is no way to send their amps a signal from an aftermarket head unit. I don't know if this applies to your Z4 or not.
The best thing for you to do would be to buy a good 4 channel amp, a decent set of 6-1/2" component speakers, and a small sub mounted in the trunk. Unfortunately, with a lot of BMW systems, you really have to replace nearly the entire thing to acheive better sound quality or output.
SOLD- Now Enjoying Honda Civic
Last edited by bigfeller; 09-16-2009 at 05:57 PM.