In our new beetle, there are actually numbered buttons on the CD stacker and you simply select the number that goes witht he disc you want to hear. In our other car, we have the same system as well as being able to push an arrow button to move up by one song or another arrow (double arrow maybe) that moves you up by a disc at a time. Have a look and see if you have the same buttons on yours. One nice thing on the single arrow button is, if you hold it down, it actually fast forwards through the song you are listening to so you can get to a particular part of the song you want to hear a bit quicker.
I hope that helps.
Yogie