E46 Convertible without H/K sound package: Where i can take audio input to amp?

Hi

I have 2004 330ci cabrio without H/K package. I have been planning if i buy little sub+amp.. install it where hk sub is located originally. Can you tell me where i can take high imputs for the amplifier in the rear? Is there some filtering for the rear speakers, there is midrange + tweeter, so there must be some kind of crossovers.



Many thanks