Listening to Music via iPad without Bluetooth?

I do not have bluetooth on my iPad. I plugged my iPad via the USB cord. It plays my iPad's podcasts. But the Prius does not seem able to play my iPad songs via music apps like Spotify. I have a hard understanding why the car can play podcasts but not songs from Spotify... maybe I downloaded the podcasts and I am only streaming songs with Spotify?
One time the car played music on my iPad's Spotify app but since then it has not worked. So I know it is possible...

Is something wrong with my iPad? my car?

Any thoughts? Muchas gracias!