Before you render your car’s cassette deck completely useless, take a sec and consider the Bluetooth Cassette Tape Adapter. Through the magic of Bluetooth it pairs your cell phone’s audio, albeit headset profile (and perhaps MP3 profile), to your car’s speakers via the cassette deck. There’s a built-in mic for picking up your voice, although I’m skeptical as to how well it would work when inserted in the cassette deck. Looks like it charges via USB.
The Bluetooth Cassette Tape Adapter is available here for $40