I have an iPhone 5s, and I can't get it to work with the aux cord in the car but my sister couldn't with her 5, then her 5s, then her SE. It doesn't work with aux cord speakers either. My phone works perfectly fine with headphones and my family uses amazon music.
Read more: https://iphoneus.org/faq/why-don-t-i-get-sound-on-aux-cords.html