2005 gmc w/ bose

I dont know which method they used to interface your factory harness but they might have reversed some wires to the line convertor which converts the high output of your speaker outputs to low inputs to be plugged into the amplifier. When the wires are reversed they seem to cancel each other out and makes the music seem lower than it actually is. So that when you turn on your rear radio box it cuts the speaker outputs off and puts the signal back in phase which makes the audio output louder. Which is  what it supposed to be. I would take it back ti the because evidentally they hooked up either just the left side or just the righht side which is wrong. You are  supposed to hook up a line convertor to oneright side and to one left side speaker output.Owner/Installer for Tricked Out Motor Works