On the Now Playing screen, the Add to Liked Songs button is very buggy in Apple Carplay while it works great for the Web player, ios app and Apple Watch app.
The Web player, ios app and Apple Watch app show a White outlined Heart icon for any song which is being played in the Now Playing screen:
Song not yet added in Liked Songs playlist - Web Player
Clicking on this icon puts the song in my Liked Songs playlist and the icon becomes green filled Heart icon: Song added in Liked Songs playlist - Web player
Clicking on the same icon again removes the song from my Liked Songs playlist and the icon color again becomes outlined white heart icon.
The same behavior is mirrored by the ios app:
ios App Screenshot
and the Apple Watch app:
Apple Watch screenshot
Now the bummer comes in Apple Carplay where instead of showing a Heart icon, a plus icon is shown:
Apple Carplay screenshot
Now, this plus icon behaves in the way that clicking on this plus icon adds the song to the Liked Songs playlist and this icon is hidden. There is no way to remove this song from Liked Songs playlist from Apple Carplay.
This plus icon also behaves erratically where tapping the heart icon in Apple Watch for the current song adds the song to the Liked Songs playlist but doesn't remove this icon from the Now Playing screen for the current song.
The idea is to make the behaviour consistent for Apple car play with Apple Watch, ios app and web player.
There should be a heart icon in Apple Carplay Now Playing screen and it should behave exactly like others.