Car stereo USB ports are not universal. Consult manual see what it says it can do. Some can only charge, others can read a flash drive but that's it, and all of them have limited codec capability, most likely can MP3, but maybe not WMA or AAC.
I've been doing this for years... trust me when I tell you just play your smartphone through a BT connection. It works, all the time, everywhere, no restriction, nice Android interface with choices of player app, the heck with finicky car stereo.