You could try a FM transmitter. It's the easiest way to get MP3 player sound into car stereos. The debate, however, is if it gives you the best sound. Probably not, but it's working without the need of installing new cables/ hardware. Try that alternative first and see if it is good enough for your needs. There are cheap ones out there nowadays, so you won't spoil to much money if the FM transmitter solution doesn't do the job.