There's a thread on this topic already:
viewtopic.php?f=22&t=16196. It's quite comprehensive and lists all the options for GEN4 audio.
As far as I know there is no way to just plug in an AUX cord to your stereo. I've got a 2004 3.0R-B and I bought an Anycar 1080D from AboutSound who are based in New Zealand. It cost about NZ$150 from memory and took a couple of weeks to arrive at Brisbane. There are also Grom units and other brands but this is the only one I have experience with (and I have next to no experience with any form of car audio).
It's basically a little box thing you plug into the back of your stereo, and on the other end are a few options depending on which Anycar 1080 you buy. In the case of the 1080D, it was an iPod connection (a larger round plug that worked with a 30-pin older styled Apple cord) and an AUX connection that you plug into any device to play music through. The iPod cord was supplied with the 1080D. Note that if you use AUX you can't use the steering wheel controls to change tracks, only volume. It's easy to tuck the box into the centre console under/next to your gearstick etc. and have the cords pop up wherever you want to keep it neat.
I'm actually looking at buying a 1080A (which has USB/SD/AUX), as I've been happy with the 1080D but I've got a new iPhone with the lightning connector so I have to play through the AUX instead of the other one. If I do end up getting the 1080A I'd be selling the 1080D so let me know if you want to grab it
