The reason I have some interest in this is because I'm one of those guys who gets the cheap radio and replaces it. I'm old school so now with all the fancy steering wheel controls, amplified antennas and back up camera it doesn't make it easy anymore. Without ripping into the car more than I want I guess I could add some better speakers and add my own dash mounted GPS NAV. Reading some other forums and watching some videos made me not want to do it. But..... check ebay, youtube with some search terms and maybe you'll hit something. I saw you mentioned RCA adapter.
2016 Silver Scion iM
2011 Nissan Frontier
2016 Subaru Forester (Wife's Car)