I just purchased a 2001 Tahoe. It has factory Subwoofer & amplifier. My 2nd row door speakers are not functioning. I've been told that it could be a damaged channel on the amplifier. I want to know with the 4 channel receiver how the signal is getting to the 6 speakers. 2 front doors, 2 rear doors & 2 in the rear pillars. Thanks for any information.