Locate the "line-in" or "MP3" port in your vehicle, most likely connected to, or near the car stereo. The "line-in" port allows you to connect a stereo audio jack cable from the MP3 player to the port.
Connect either end of a stereo audio jack cable to the player and the line-in port.
Locate and press the button on your car stereo that switches to the "line-in" port. Play a song on your MP3 player; the song should be audible through the car's speakers.
Locate an MP3 FM transmitter (see Resources).
Connect the transmitter to the MP3 device's headphone or line-out port. Power on the transmitter if it has an on/off switch.
Examine the transmitter to determine which FM station to tune your car radio to. Usually, the lowest FM station is a good idea since it is not likely to be used by a radio station.
Tune your car radio to the designated FM station, and press play on your MP3 player.
Obtain a cassette tape adapter (see Resources).
Plug the adapter's plug into the MP3 device.
Insert the cassette tape adapter into your car stereo and press "Play."
Press "Play" on your MP3 device to stream audio from the device to your car's speakers.