Do you have a satellite radio or MP3 player that transmits sound to your car radio or home stereo?
There are many great audio gadgets available today that include a small FM transmitter and work by sending a signal to any nearby radio that's tuned to its frequency.

These devices work best if you choose an unused frequency on the FM dial. It's also best not to pick a frequency that's too close to a strong FM station. You can use this page to find vacant frequencies in your area that are best suited for use with your audio device.