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Wow, you are going to need a shop with a tech that knows how to use an advanced multimeter or oscilloscope to trace this one. Somehow EMF is being transmitted through the radio and speakers from the defrost circuit. You could start replacing wires , switches and connectors (one at a time) but you'll soon run out of time and money unless you guess it right. Perhaps it's in the window itself?
First thing to get would be a Chilton's or Haynes manual (if one is available) and find a circuit schematic. BMW uses brown wires for ground.
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