Step 1: Check for USB port
Make sure that your vehicle has a USB port and supports USB mass storage devices.
Step 2: Connect your Android phone
Connect your Android phone to your car's USB port using a micro-USB cable. You should see a small USB icon appear in your notification bar.
Step 3: Select the USB notifcation
Pull down your notification bar and select the "USB Connected" entry.
Step 4: Mount your SD card
Select the option to mount your SD card as a USB storage device and wait for the phone to confirm.
Step 5: Select USB audio source
Using your car stereo's interface, select USB as the audio source. The stereo should then search for audio files on your Android's SD card.
Step 6: Enjoy your music
Once your songs have been indexed, you'll be able to listen to them through your car's speakers. Some systems will organize the songs by metadata, but most will just allow you to navigate your folder structure.