In my 04 Silverado we tapped into the wires running to the back speakers with a high-pass converter to RCA's and ran them to the amp for "sound". I am a bit nervous though with my 08 because of the On Star and side air bags. Can I tap into the rear door speakers on the 08's like I did with the 04? I'm not wanting a loud system. I have a simple little amp and a 10" sub that just rounds out the stock sound system. I want to keep the factory deck which of course doesn't have RCA Preouts .... I assume. Anyone that has done something similar, please chime in with suggestions.