I have a non-DSP, 10 speaker system so my install is a bit different than yours (especially on what wires after the amplifier to tap into), but for the easiest way to wire up an aftermarket amp, go to Bavarian Sound Werks website for the install (click on the "Install Info" tab, and it will guide you on what wires to tap):

X5 Subwoofer / Sub System for BMW X5 | BSW (link for both DSP and non-DSP installation)

Or just click on the PDF for the DSP stereo (which if my links work right, it should work):

http://www.bavariansoundwerks.com/pr...System_DSP.pdf

They seem to have a great OEM fit subwoofer to take place of your DSP setup's subwoofer location and will guide you on how wire up your DSP-equipped stock stereo. You don't need to buy a subwoofer/amp from them, but their instructions are very clear on what you need to do.

A few things to take note of: for your line output converter, you MUST tap the wires going from the factory amp to the speakers (high-level inputs) as BMW uses a wierd crossover system. You should NOT tap the wires coming from the radio to the amp (low-level) to your line output converter as you will lose the fading, volume, balance, and DSP functions--based on what I've read here on Xoutpost.