you can use an FM transmitter to stream mp3 to your radio. I actually still have mine from before I sold my laptop... it plugs into the USB port, and then you tune your radio to like 88.1 FM and it'll play. The sound isn't perfect, but it is ok... better than regular radio since its transmitting so close. This is definately the simplist solution... and it requires no wiring. You can also stream it to your home stereo with this for parties and whatnot. If you want mine, it is a really good one (not a cheap iROCK) made by LineX. It was like $90 about a year ago... I'd sell it for like $40.
As for direct wiring, it might be a little tricky... you will need a laptop with RCA audio plugs and your head unit will need an AUX port. If you have an aftermarket HU that doesn't have an aux port, but it does have cd changer controlls, check out
http://www.xmfan.com which will convert your cd changer controlls to rca jacks. If you have the stock HU, they also sell a kit that will give you RCAs, but it's more expensive.
Hope this helped.