Depending on the smart phone you have, if you can acquire root access on it, you can download a stick mount app. Do that and it will act as a secondary sd card, of sorts, and from there, get on your mp3 player on your phone and rescan your folders, or if you have Poweramp, go to Folders and Library and change the directories to the usb stick. Once it rescans all your music will be on your phone and then you can connect your audio jack from your phone to your car and play all your music.

I had to do this for my friend's Kindle. I rooted it like I mentioned above and then downloaded stick mount pro (kat it) and once it detected it I used the media player on her Kindle (just the default one) and it loaded everything. Now everytime she goes out anywhere she just carries the usb stick with her and when she wants music she just plugs it in and the audio jack and she has her music. If you need help rooting your device either pm me or look it up yourself on XDA Forums.