This is a work around for those with Apple Watch or iTunes Match Subscription.
From my observation Siri on CarPlay will only search the Apple Music Subscription for the playlist.
- Apple Watch: For those with Apple Watch, this method bypass the Siri interface via CarPlay, simply raise the watch to your lips and say: “Hey Siri, play playlist x” and Siri will play flawlessly. Just have to make sure the “‘’Listen for “Siri” or “Hey Siri”’’ is activated on your watch. Search the web, if you don’t know how to do this;
2. iTunes Match Subscription: This is a more complicated work around and make sure you do this on WiFi; or else, the iPhone will gobble up your mobile data.:
- On your Mac or PC, on your iTunes app, Goto the “Settings” select “Sync Library”, then, “OK”. Following that - Click “File” - “Library” - “update Cloud Library”. This takes a while to complete.;
- Delete your music library from your iPhone :- Goto “Settings” - “General” - “iPhone Storage” - “Music”; then, scroll down to the middle of the page for ”Edit“, next to “All Songs” click on the red circle with “-“. Wait for a few minutes for the iPhone to delete all songs from the device.
- Goto “Settings” - “Music” - “Sync Library” toggle that option on.
- Open the Music app, from here you’ll find all your music library and playlist. At this point, the music library is stored online. Select the playlist that you use most often, you will find “+” on top of the screen, next to the name of your playlist. Click on the “+” sign to download the songs onto your device.
So, bear in mind, other then the playlist you have downloaded, the rest of the songs reside online, should you play all music while using CarPlay, you are using Mobile Data. Download as many playlist as you want and only ask Siri to play playlist.
From now on, until Apple Developers fix the Siri on CarPlay playlist search parameter, do not connect your device to your Mac or PC. Any music from iTunes to the devices will render playlist unworkable again.
Good Luck!!