Thanks, JimCost2014:
Not sure what radios the antenna in the shark fin serves, but my AM/FM radio is working just fine. Do you know if that means anything about the shark fin and the XM not working?
BTW, thanks to YouTube, I opened up the radio compartment and checked all cable connections. no problems there. While doing this, I discovered that the XM module in my 2014 Sierra is located behind the screen and above the AM/FM radio, NOT next to the glove compartment as claimed by several YT videos.