2000 Audi A4 - Upgrade rear amplifier and speakers

I have a 2000 Audi A4 with stock head unit and speaker/amplifiers. In the trunk, the wiring harness plugs into an amplifier which is located under the left rear speaker. I would like to keep my existing head
unit and wiring harness and add the following upgrades (Eliminating the stock rear speakers and amp):
-- Carver KMOS 2150 2-Channel Amplifier -- Boston Acoustics 6.5 components speakers to the rear deck.
I plan on running 12V+ with a fuse block right from the battery and ground the amplifier to the frame. Is there an aftermarket wiring harness kit available anywhere that will allow me to plug into the stock wiring harness (in the trunk), in effect giving me the available line-level left-right signal and remote power feed needed to run my amp (without cutting the stock wires)?
Okay, if I have to cut the wires I will. So I guess my other question is, has anyone out there done a setup similar to this before? And if so, could you tell me what each colored wire is for?
Thanks, Hertz
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If you can find the speakers you want from them, Crutchfield will include wiring adapters and installation instructions free of charge with a purchase. My question for you is, why upgrade the rears and leave the fronts alone? The best way to do things would be to put better speakers up front and leave the factory rears in for low-volume fill. In addition, the amp you want to disconnect may be responsible for driving the front speakers as well as the rears, so disconnecting it would eliminate power to the front speakers. Beyond that, if the Symphony system in your A4 works like the Bose system, it may have smaller amps attached to each speaker, which requires bypass work and a lot of knowledge of what you're doing.
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http://vader.inow.com/~drbob/carstereo.html
this might help you...
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