The problem persisted.
Finally in the middle of a mind-wandering commute daze it occurred to me that my wife's phone had no songs in iTunes . She only listens to music via YouTube (don't ask). "You don't suppose...".
Yup. I put one song in her iTunes library and problem solved.
Literally yesterday I read about this "fix", so i re-downloaded the Music app and put just one song on it... yep it works!
Car players apparently assume you will only ever play music from the "default" library/app so when you connect it and it sees an empty library index, it assumes nothing can ever be played and just stops you there.
Very dumb.
I'd conjecture it's a matter of how thoroughly apps got debugged by the vendors.
Never thought I'd be happy to see Carplay again with its 3-ring circus of UI tomfoolery, but there it is.
For example, my Google Home can play the Beatles on Youtube, but it fails with Google Play.
Second, I wonder how much PEBKAC issues like this cost apple per year in replacement devices?
No memory card inserted? No UI difference between "No Ethernet plugged into XBox" and "XBox connected correctly to LAN".
Insert memory card? Network magically appears.
(I may have almost taken a sledgehammer to the hardware after I stumbling across this issue while setting up a PAX game room. And no, the memory card is not used for LAN play.)
So you can't charge your phone in peace or use the car stereo for turn-by-turn navigation or anything else without also playing some music in the background.
It's absurdly frustrating, and I've joked with my wife about sticking a long silent track on her iPhone more than once. (I have an Android that the car just refuses to connect to at all.) I'm going to send her this article.
I was eventually able to remove it, but for a few weeks, auto-play music (and Bono) were the bane of my existence.
Oh my God, so that's why I always hear "Andrea Bocelli" when I plug my phone into my car. That was always bugging me, but not so much to actually look it up. Turns out many people have the same problem and this 'song', cleverly named "A a a a a Very Good Song" somewhat solves the problem, "effectively giving users 9 minutes and 58 seconds to choose their desired playlist and songs before starting their journey by car". Simple, yet brilliant.