On my phone I have both Spotify and Plexamp. For reasons I don’t understand, I am able to ask Siri to play music from Plexamp outside of the car, but I cannot do this when connected to CarPlay. Every time I request music it says it cannot find that speaker. ¯_(ツ)_/¯
Ok. After I posted and thought about I wondered if Plexamp could even be the issue as it is happening before it ever “reaches Plexamp” and seems to be an OS issue. I’ll continue to dig into why iOS seems to think Spotify is the only option for CarPlay