micky wrote:

My new used car from 2005 has a CD that will not play an MP3 CD, but I
can buy an adapter that will add an AUX input and a USB input to the
radio. (Since the radio has a CD changer jack that is not in use, and
this is designed for that shape of jack.)

Long after I ordered one for $18, it suddenly dawned on me that if it
can't play an MP3 CD, it won't be able to play an MP3 file from a
USB-connected flashdrive. That's true, is it not????

It's amazing how long it took me to think of this problem, at least a
week.

Staying in touch with a particular phone operator at your provider's help desk is key. MP3 players (and now the cloud) sometimes don't do what they're not specifically programmed to do and help desks can go over that with you.
If you don't want to ask your phone or device service provider what kind of car radio you should have in your car, then you could always go to the junkyard and into a ruined car and take out a radio system that may still be good.