Fortunately I've not had an issue or check engine light. When I first bought my 2008 in 2009, I remember scanning for codes and my scan tool worked. I'm wondering if yanking the AM radio and installing the aftermarket would keep the scan tool from communicating? This is a work van w/ no Onstar. I remember using the adapter harness (this is 10 years ago) on initial install, but then there was a speaker wire issue to one of the doors and I may have rewired it. I remember that it came w/ a relay so you could hear the turn signals and went skip that , sounds like a gift to me. Sound plausible?

 

Unless your radio connects to the data bus, it's unlikely.

Did you check the fuse? It think it's shared with cig lighter. Is there power to the connector?