Hmmm, ok. Thanks for the response. With all my googling and searching, I kept getting an adapter from PAC that showed up when searching for IOB steering wheel controls. Looks like this adapter is specifically for the IOB (and IOA) radio builds, which was giving me hope.
https://pac-audio.com/catalog/radio-replacement/rp5-gm52-har?search_query=rp5-gm51&results=4
I'm getting keyless entry added by the dealership on Friday. While I'm there, I'll venture off to the service department and see if they know anything. Not holding my breath.