Hello,
The GM21SR Is designed to be used with any aftermarket radio. To get the most current application guide information, please contact Scosche Tech Support: ...
www.scosche.com/ecom/download/GM21SR.pdf Thanks.

Do you have On-Star in your 2007 Monte Carlo?

Is it a Bose equipped from factory?


According to my Scosche book

GM21SR is for OnStar equip Monte

GM22SR is for standard non Onstar Monte
non-Bose system only

====
As quick check disconnect the smaller power connector to the GM21SR,

Wait 2 minutes.

Key out

Reconnect the smaller power connector

key to on only (not staring) for 5 seconds and key out

Re-set the Onstar volume and the chime/dimmer programing again.

Hello,

Check to ensure the radio volume is at mid or at an higher volume, perhaps, you can also check if the radio is not on mute, see the procedure to wire your radio,

Car Radio Battery Constant 12v+ Wire: Orange
Car Radio Accessory Switched 12v+ Wire: Yellow
Car Radio Ground Wire: Black
Car Radio Illumination Wire: Gray
Car Stereo Dimmer Wire: Light Brown
Car Stereo Antenna Trigger Wire: Pink
Car Stereo Amp Trigger Wire: N/A
Car Stereo Amplifier Location: N/A
Car Audio Front Speakers Size: 3 1/2′
Car Audio Front Speakers Location: Dash
Left Front Speaker Positive Wire (+): Tan
Left Front Speaker Negative Wire (-): Gray
Right Front Speaker Positive Wire (+): Light Green
Right Front Speaker Negative Wire (-): Dark Green
Car Audio Rear Speakers Size: 4′ x 10′
Car Audio Rear Speakers Location: Rear Deck
Left Rear Speaker Positive Wire (+): Brown
Left Rear Speaker Negative Wire (-): Yellow
Right Rear Speaker Positive Wire (+): Dark Blue
Right Rear Speaker Negative Wire (-): Light Blue

Take care.