I am troubleshooting a Radio module error U18AF-04. One clue is that OnStar does not work and I get an occasional SD card error. Where is the OnStar module located so I can check wiring harness for issues?
Take it in to dealer and have them give you a loaner. I think one module is in the back and one is under the dash. I know the radio is under the dash by the glove box as i had mine just replaced because it wouldn't understand voice commands.