A test that can be done is to bypass the phone modules fiberoptic cables. To do this you would need a fiberoptic coupler for the cable. I have them in stock. Let me know if you want more help.

Cleeve

Cleeve

I don't believe the 94 uses fiber optics or at least my 94 E320 Cab doesn't.
All hard wired included the trunk mounted changer.

The phone mute circuit operates to ground, so if you pull the radio and clip the yellow/black wire on pin A3, it should permanently disable the mute function.

Ed A.

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1971 280SL ROADSTER
1988 300CE TWIN TURBO WIDEBODY
1994 E320 CABRIOLET
1999 C43 AMG
2005 G55K AMG
2008 CLK63 AMG BLACK SERIES