How so you ask? Start by purchasing an iSonTalk Microphone Headphone Adapter and a cassette-to-3.5mm adapter. Next, plug everything in into its respective receptacles. Crank your tunes and take a spin around town. When a call comes, simply click the mic’s answer button, the music stops, and the caller should be heard over the car’s stock speakers. I’m not sure on the feedback implications of the mic picking up the caller’s voice (it’s designed for headphone use), but if all goes right it’s a super cheap yet slick iPhone integration into any car with a cassette deck, and it’s only $20 (plus whatever the cassette adapter costs you).
Available here for $20.