The most common place for the box is either under the passengers side front carpet or under the pass front foot rest. To access it take the pass door threshold trim out by pulling straight up then the whole carpet section will come out in one piece.This should expose the module and make it easier to trace the problem and re wire. By the way did you remove the rubber antenna connector cover on your phone?
The phone mute wire on nokias is a ground and will directly connect to the yellow black wire on the back of the becker radio. Let me know if you have any more questions.

Scott@lajollaaudio

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The Mercedes Specialists Since 1986