Okay, I did a little research... I Googled "Sony CDX-M600 iPod" and found this post in an iPod discussion thread...
Mike Kelleher
I've got a Sony head unit (a 2000-vintage CDX-M600) and Sony's XA-300 Aux Input Adapter to get RCA auxiliary inputs (a $100 option that's not necessary with Sony's newer head units). Coupled with that I bought a used Belkin iPod Auto Kit on eBay ($18 shipped!), and a cigarette lighter socket kit from Radio Shack ($6-10). I tapped into the power going to the head unit (for best engine-noise rejection), and installed/buried the lighter socket and Auto Kit plug-end in my center console.
So ultimately I just have the Auto Kit's dock cable coming out of my center console to plug the iPod into. I select the AUX input on the head unit and I'm good to go. I don't get track control with the head unit, but the iPod is right there on my center console. I don't have to worry about upping the iPod volume like I did when using the headphone jack (or reducing the volume before plugging in my headphones...ouch!), and the best thing is that since the iPod loses charge power when I turn the car off, it pauses itself just like with the $150-200 BMW, Denison, or Alpine kits.
Overall it's a very neat installation for my car, since I was able to bury the Sony Aux Input box and the Belkin Auto Kit's cigarette-socket end in my console and the only visible part is the end of the Dock cable. I just need to find a source for blank switch covers so I can drill it and make a neater installation where the cable comes out of the console.
...if there's enough interest I'll dig back in and document what I did.
In this post there were two links... one to Crutchfield for the $100 AUX input adapter...
http://www.crutchfield.com/S-Qt7g7tJ...900&I=158XA300
the other to the iPod power adapter by Belkin.
http://catalog.belkin.com/IWCatProdu...duct_Id=149006
If I understand the post and the info on Crutchfield it looks like you may need this to use something other than a CD-changer as an AUX unit. But then I also found this...
Selecting The Auxiliary Device: On the Sony changer controller head unit, press "SOURCE" repeatedly until "CD" is selected. Press "MODE" repeatedly until the device you want to listen to is selected. The head unit will cycle through each unit in the following order: CD1 (CD/MD changer) > CD2/AUX1 > CD3/AUX2 > CD4/USB. When CD1 is selected, the CD/MD changer will automatically start playing. When any other source is selected, playback of that unit will have to be manually started and the volume adjusted. As the volume settings of each connected device may differ, the volume of each device should be set to a similar level as the CD/MD changer.
You may want to try this to see if you can get the AUX1 to show up. I get the feeling that you'll have to either upgrade your headunit or plunk down $100 for this adapter.
Again... good luck,
JD