I keep this with my iPod in case someone has a tape player in their car. Comes in handy!
I've always used this cassette adapter for my iPod since my car has a tape deck. These sell for about $17 and give perfectly good sound quality over your car speakers. The ideal iPod adapter for road trips, since they're inexpensive and easy to use. Paired with an iPod car charger that plugs into the cigarette lighter, you've got an unbeatable duo.