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According to http://www.installdr.com/Harnesses/GM-Wiring.pdf the connector for the rear speakers is separate from the front speakers. If the connector for the rear speakers is loose or disconnected, the rear speakers would not work while the front ones will. The amp in the head unit (dashboard radio) powers the front door speakers. The rear door speakers and roof (pillar) speakers in the rear are powered by a second amp under the drivers seat. Either this amp is bad or is not hooked up.
Radio Constant 12V+ Wire: Blue/Yellow Radio Switched 12V+ Wire: Gray Radio Ground Wire: Brown Radio Illumination Wire: Green Radio Dimmer Wire: N/A Radio Antenna Trigger Wire: Black/Red Radio Amp Trigger Wire: None Front Speakers Size: 5 1/4? Front Speakers Location: Doors Left Front Speaker Wire (+): Pink Left Front Speaker Wire (-): Purple Right Front Speaker Wire (+): Light Green Right Front Speaker Wire (-): Blue Rear Speakers Size: 6 1/2? Rear Speakers Location: Rear Deck Left Rear Speaker Wire (+): Black Left Rear Speaker Wire (-): Yellow Right Rear Speaker Wire (+): Red Right Rear Speaker Wire (-): White
Radio Constant 12V+ Wire: Blue/Yellow Radio Switched 12V+ Wire: Gray Radio Ground Wire: Brown Radio Illumination Wire: Green Radio Dimmer Wire: N/A Radio Antenna Trigger Wire: Black/Red Radio Amp Trigger Wire: None Front Speakers Size: 5 1/4″ Front Speakers Location: Doors Left Front Speaker Wire (+): Pink Left Front Speaker Wire (-): Purple Right Front Speaker Wire (+): Light Green Right Front Speaker Wire (-): Blue Rear Speakers Size: 6″ x 9″ Rear Speakers Location: Rear Deck Left Rear Speaker Wire (+): Black Left Rear Speaker Wire (-): Yellow Right Rear Speaker Wire (+): Red Right Rear Speaker Wire (-): White
You either have accidentally Disconnected the Wires to the rear speaks or you have the Fade on the radio set to the front.
Radio Constant 12V+ Wire: Blue/Yellow Radio Switched 12V+ Wire: Gray Radio Ground Wire: Brown Radio Illumination Wire: Green Radio Dimmer Wire: N/A Radio Antenna Trigger Wire: Black/Red Radio Amp Trigger Wire: None Front Speakers Size: 5 1/4″ Front Speakers Location: Doors Left Front Speaker Wire (+): Pink Left Front Speaker Wire (-): Purple Right Front Speaker Wire (+): Light Green Right Front Speaker Wire (-): Blue Rear Speakers Size: 6″ x 9″ Rear Speakers Location: Rear Deck Left Rear Speaker Wire (+): Black Left Rear Speaker Wire (-): Yellow Right Rear Speaker Wire (+): Red Right Rear Speaker Wire (-): White
Radio Constant 12V+ Wire: White/Green Radio Ignition Switched 12V+ Wire: White/Red Radio Ground Wire: Black Radio Illumination Dimmer Wire: Red/Black Radio Antenna Trigger Wire: N/A Radio Amplifier Trigger Wire: N/A Front Speakers Size: N/A Front Speakers Location: N/A Left Front Speaker Wire (+): Blue/Green Left Front Speaker Wire (-): Gray/Black Right Front Speaker Wire (+): Red/Green Right Front Speaker Wire (-): Brown/Black Rear Speakers Size: N/A Rear Speakers Location: N/A Left Rear Speaker Wire (+): Blue/Yellow Left Rear Speaker Wire (-): Gray/White Right Rear Speaker Wire (+): Red/Yellow Right Rear Speaker Wire (-): Brown/White
Remove the covers to your 1997 radio speakers. Remove the radio speaker retaining screws. Remove the wiring harness from the back of the radio speakers. Reverse the process to install your new radio speakers.
Radio Constant 12V+ Wire: Orange Radio Ignition Switched 12V+ Wire: Yellow Radio Ground Wire: Black Radio Illumination Wire: Gray Radio Dimmer Wire: Brown Radio Antenna Trigger Wire: N/A Radio Amplifier Trigger Wire: N/A Front Speakers Size: 6 1/2″ Front Speakers Location: Doors Left Front Speaker Wire (+): Tan Left Front Speaker Wire (-): Gray Right Front Speaker Wire (+): Light Green Right Front Speaker Wire (-): Dark Green Rear Speakers Size: 6 1/2″ Rear Speakers Location: Rear Doors Left Rear Speaker Wire (+): Brown Left Rear Speaker Wire (-): Yellow Right Rear Speaker Wire (+): Dark Blue Right Rear Speaker Wire (-): Light Blue
Radio Constant 12V+ Wire: White/Green Radio Ignition Switched 12V+ Wire: White/Red Radio Ground Wire: Black Radio Illumination Dimmer Wire: Red/Black Radio Antenna Trigger Wire: N/A Radio Amplifier Trigger Wire: N/A Front Speakers Size: N/A Front Speakers Location: N/A Left Front Speaker Wire (+): Blue/Green Left Front Speaker Wire (-): Gray/Black Right Front Speaker Wire (+): Red/Green Right Front Speaker Wire (-): Brown/Black Rear Speakers Size: N/A Rear Speakers Location: N/A Left Rear Speaker Wire (+): Blue/Yellow Left Rear Speaker Wire (-): Gray/White Right Rear Speaker Wire (+): Red/Yellow Right Rear Speaker Wire (-): Brown/White
Radio Constant 12V+ Wire: White/Green Radio Ignition Switched 12V+ Wire: White/Red Radio Ground Wire: Black Radio Illumination Dimmer Wire: Red/Black Radio Antenna Trigger Wire: N/A Radio Amplifier Trigger Wire: N/A Front Speakers Size: N/A Front Speakers Location: N/A Left Front Speaker Wire (+): Blue/Green Left Front Speaker Wire (-): Gray/Black Right Front Speaker Wire (+): Red/Green Right Front Speaker Wire (-): Brown/Black Rear Speakers Size: N/A Rear Speakers Location: N/A Left Rear Speaker Wire (+): Blue/Yellow Left Rear Speaker Wire (-): Gray/White Right Rear Speaker Wire (+): Red/Yellow Right Rear Speaker Wire (-): Brown/White
Radio Constant 12V+ Wire: Blue/Yellow Radio Switched 12V+ Wire: Gray Radio Ground Wire: Brown Radio Illumination Wire: Green Radio Dimmer Wire: N/A Radio Power Antenna Trigger Wire: Black/Red Radio Amplifier Turn On Trigger Wire: N/A Front Speakers Size: 3 1/2″ Front Speakers Location: Dash Left Front Speaker Wire (+): Pink Left Front Speaker Wire (-): Purple Right Front Speaker Wire (+): Light Green Right Front Speaker Wire (-): Blue Rear Speakers Size: 5 1/4″ Rear Speakers Location: Rear Deck Left Rear Speaker Wire (+): Black Left Rear Speaker Wire (-): Yellow Right Rear Speaker Wire (+): Red Right Rear Speaker Wire (-): White