Depends what you're inputting to really.
You can get special adapters that "pretend" to be cassette tapes, but plug into a headphone output of a CD/Minidisc/other player, to allow you to play through a car's speakers.
There are other methods, if what you're connecting to has a "line input", then you can connect anything with a "line output" to it (headphones use the same electrical characteristics as a line out, so can be used interchangably)