Eliminate and test whats wrong.
Use pen stick with music on it - no larger than 32GB (i tested 64GB and it was invisible for the car ). If it works first canle is ok.
your phone - what phone u are using ? cable for the phone, if u bought cable to use and leave it in the car, test it with PC can u transfer data using that cable, ase some cheap ebay phone cables are made only to be able to charge the phone.
if u have Android phone there is some options for type of connection you can change while phone is connected to USB ...