If your car audio system has a USB port to play MP3 music, you'll need to copy the MP3 files to a USB thumb drive and plug it into the USB port of the car audio system.
What if your car audio system doesn't support the above 2 ways of playing MP3 music?
Well, you can convert and burn your MP3 files into an audio CD, which can store about 10 songs, and play it with your car CD player.
There also exist a kind of FM Transmitter that plug into your car's cigarette lighter socket, which you need to copy your MP3 files to an SD card and slot it into the FM Transmitter. Then, you can tune your car radio to the same frequency with the FM Transmitter in order to listen to the music.
But I have a better choice. I use the kind of FM Transmitter as shown below which can turn your handphone, MP3 player, or any kind of gadgets that support audio output to a universal 3.5mm headphone jack.
This FM Transmitter turns your gadget into a mini radio station with broadcast range of about 15 meters only. It supports devices with the 3.5mm headphone jack including handphone, MP3 player, voice recorder, tablet, laptop, etc.
The audio transmitted out can then be received and played by any radio device within the range. This could be your car radio, a normal radio, a Hi-Fi, a handphone with FM radio function, etc.
This FM Transmitter is able to transmit high-fidelity stereo sound with the digital PLL (phase locked loop) technology. It supports full range of FM transmitting frequency (87.5MHz-108.0MHz). It has stereo isolation of >= 38dB.
You can use it to output any kind of audio from the connected device, with audio frequency range of 100Hz-15,000Hz.
It has its own built-in 120mAh Li-ion battery, which can be recharged with its micro-USB port.
Its usage is similar to the cigarette lighter FM Transmitter. Just tune your car radio to the same frequency with this FM Transmitter in order to listen to the music.
Pretty cool isn't it?