I have a 2004 Tahoe w/o the Bose system. No amp. The rear door speakers are much quieter than the front. I have to turn fader way to rear to get balanced sound. I think this happened after installing aftermarket deck (Alpine CDA-9851). I even replaced the stock rear speakers (with Alpine SPS-130A) and the problem continues. Is this a bad wiring problem, impedance problem or what? Anybody have any ideas?