factory speaker wiring 02 avalanche

Hello, I have a 2002 Avalanche that I want to install a aftermarket head unit into.  I have the GM "89-up" harness but there is another connector to plug into in the dash.  The factory deck does not work without it either so it must be needed.  I heard that it is ued for the factory amp.  I would like to either purchase the additional harness needed or if someone know the best location to tap into the factory wiring after the factory amp and use deck power for the speakers.  Thanks

Hello, what you can do is bypass the amp all together. With the new deck it should have the speaker wires in the clip. Cut the factory wires leading to the speakers and then run better speaker wire directly from the deck to the speakers. This will eliminate guess work. www.carLOUD.com