I would say that your analysis is mostly spot on regarding the three independent variables: the Android OS, Pandora phone app and Pandora head unit app. As you said, the head unit app vaires from manufacturer to manufacturer but is rarely updated. That said, the head unit app on a Pioneer head unit today is probably not the same as the one on a head unit manufactured several years ago. So we have additional head unit variations beyond just the manufacturer. The phone app changes frequently, but that is the one variable that users can manually control. Android also changes frequently, and like the head unit app, Android on one device is not the same as on another device. So assuming that your Pandora phone app was working with your head unit, where do you look to find what broke it? If you are lucky, like me, the only problem is the Pandora phone app. I installed an old version and everything works fine now. But if a new Android update breaks it, I'll just be out of luck. I suspect that is what is happening to those folks who cannot restore funcionality by installing an old version of the Pandora phone app. I suspect that the old version of the phone app is not compatible with the current version of their phone's Android OS. Some of us are also fortunate in that our head units support Android Auto. It does require me to plug in my phone, but for long trips I tend to use it to also access Google Maps or Waze while listening to Pandora.
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