I just got a new Android phone. I tried plugging into my auxiliary cord to play in my vehicle as I've always done. The music won't play thru the car speakers, but will play on the phone. Cord is new & yes, plugged in.
Plugging into an aux input is the same as plugging in a headset sin't it so I somehow doubt that is anything to do with spotify. You might want to check your phone and car audio settings and try a different cable if it plays through earphones.