Buy a tape adapter. You put the tape-like part into the tape player of the car. From that part there's a cable with a cinch connector, which fits into the iPod, iBook or any other device which has a headphone plug.
Some stock ones allow you to connect directly though the radio's AUX feature/mode. Some have a proprietary connector, others use mini-stereo jacks. It all depends on a lot of factors. Aftermarket ones are generally easier to deal with, at least the mid to upper range ones (Clarion, Alpine, Kenwood, etc.).