I think my car have the same setup as yours. I bought this one:
http://www.yatour.net/shop/?type=list&classid=1
Well... it's a mainland Chinese brand, I got it from a ebay like Chinese shopping site. They have different cables that connecting to different brand/model of the radio. What I did was pulling the radio out, pulled out the connector to the disc changer and plug the cable from that device back to the same position. I route the wire from the back so that I put the device inside the storage box in front of passenger side.
Just put the SD card with songs inside and that's it. If you don't like using the SD card, you can plug in a USB thumb drive or using 3.5mm audio cable as well.
By searching the website for you, just notice they even have a new model that allow you to easily switch over their device and the CD changer again...
You may try to find something like this on ebay, believe they are not the only manufacturer doing the same stuff.