Where to pick up a source for bass amp?

[Gwegz320]

Hi everybody.
First, I don't have a X5, but a 320Cd E46 from 2005 with MKIV Nav system and Harman/Kardon audio.

i want to know, is there a difference between Harman/Kardon audio system and the BSW audio system?

In my previous car i was a double 36' sub with a hard instalation and i waqnt to re use only one 36' phoenix gold for my BMW. Because the Harman/Kardon begin to made some vibration in my door etc, I want to lowest the bass level on it and leave the job to my subwoofer.

Ok it's cool but I need some help. Where can i pick up the sound to input in my amp?I know that all is in the back of the vehicle, but i don't know where to look.

So, i need a source to pick up the sound to enter in my amp with cinch, can you help me?

Thanks in advance