You can buy a radio transmitter or a tape adapter – both can connect to any mp3 players headphone jack. Staples probably has both for under $20. The tape adapter is more reliable, radio transmitter can be hard to use depending on where you live. But you can just pop the tape into your car, the wire still sticks out – plug that into your ipod/zune/sansa/etc and hit play.
Go to Staples and search MACALLY POD TAPE Cassette Tape Car Adapter