Tell you what would be a really cool invention, in fact I think I've just invented it.... A cassette mp3 player, not one of those plug in adaptor jobs, but an actual mp3 player that is the same size and shape as a cassette, has built in battery and the little device that sends the signal out to the tape head, it uses the tape drive wheel to work out what to do, so when you press play on the tape deck it starts playing, when you press fast forward it skips tracks etc... Could have normal buttons and a headphone socket for use outside the car.
What do you think ???
The other thing though is have you tried the FM transmitters ?, plug into your mp3 player and it transmits the signal on a standard FM radio band so you can pick it up on your old car stereo, I've got one and they work really well, sometimes tune all the radios in the house into the signal and get the same tune all round the house.