Bluetooth FM Transmitter for Vehicles for $13.
Plug in to your car's power port, pick an FM frequency to wirelessly stream music, GPS/Maps directions, sports, or anything from your phone to your car's stereo speakers. Once the device has been paired with your phone's Bluetooth, it will play the audio on your car's stereo. The volume controls in your car will control the sound, just like a radio station.
This and other similar vehicle FM transmitter kits normally sell for at least $18.