2004 CLK 320 (209.3) suddenly lost audio output while playing a CD.

Radio turns on and off as commanded, illuminates, changes and displays channels, but indicates "No CD Changer" even though one is installed. No audio output, regardless of volume setting, in AM, FM, WB or tape modes.

Swapped out existing changer with one of known quality, but still displays "No CD Changer" and has no audio output. Fuse in trunk-mounted amplifier is intact.

Any idea as to the problem?? Thanks.

Also, display in Instrument Cluster reads:
Audio ------

and the radio head push buttons are no longer illuminated when the head lights are on.

Now, when turned on, radio displays "PROD" after approx. 30 seconds.

Data card indicates the Audio 30 head unit is by Alpine.

Any ideas?