They are called cassette tape adaptors they plug into the head phone jack of your mp3 player and the other end to your tape player in your car. You can find them at almost any electronics store.
How can I use my MP3 player in my car. I've heard that the easiest way is to buy some sort of adapter that plugs into your tape deck and allows you to play the songs off your MP3 player through your stereo. Is this the best way, and where can I find this at?