Audio signal BMW 520d E60 where do get it from?

A quick question to the BMW car audio gurus... My car (BMW E60 520d 2009) have the standard sound system (no amplifier, with 6 speakers and no twitters) so where is the best place to take the signal from to be used as an input to my audison bit ten. I read somewhere that the underseat subwoofers are fed with an unfiltered signal (full range) is this true? so can I connect through an extension the signal that goes to the underseat subs to my audison DSP?

Thanks for your help in advance!