1999 S80 CD changer

[mn11569]

I just purchased a 1999 S80 that has a 6 CD changer in the trunk. In slot 1 is the CD for the GPS system that seems to be working fine. The unit has power because it let me eject the CD cartridge and insert 5 CDs. What I can't figure is how to select the CDs in the changer. The stereo seems to only let me use the CD in the dash. I am trying to use the knob in the stereo where you select the radio stations however it doesn’t function when a CD is in the dash and playing. Suggestions please.