cheapest option for decent quality is to get a tape adapter with a headphone jack and plug it into the radio...ugly, but it will work for cheap
a better option would be to get an adapter that will work with the headunit. i have an 05 scion tc and i about $60 for an adapter (auxillary input convertor) that basically tricks the stock cd player into thinking that it has a cd changer attached, but instead allows me to switch to the cd changer and use it as an input. i have my sirius sportster 2 hooked up to it. i also have the headphone jack split, so that i can plug it into my ipod. this option gives you the best possible sound.
check this out:
http://www.logjamelectronics.com/pievwaux.html
read everything though, because if you have a cd player - it will require a special kit. i bought mine from this company and called first, they were pretty knowledgable