Hi crutschow,
Inspired by your above idea here is a variation that may work. first have a wire fairly close to the wire that you are trying to detect the presence or absence of 12 volts connected to the zero volt reference of the 12 volts. Then have a rotating disk that one half was covered in conducting material placed so that the conductive part was near the 12 volt wire for 50% of the time and the zero volt wire for 50 % of the time. Then connect the conductive part of the disk (Via a slip ring.) to a high input impedance amplifier. (The amplifier ground being connected to the zero voltage reference.) I think there would be an AC output if there was a voltage difference between the wires but no output if there was no voltage difference. This would probably need screening to make it reliable.
Les.
Inspired by your above idea here is a variation that may work. first have a wire fairly close to the wire that you are trying to detect the presence or absence of 12 volts connected to the zero volt reference of the 12 volts. Then have a rotating disk that one half was covered in conducting material placed so that the conductive part was near the 12 volt wire for 50% of the time and the zero volt wire for 50 % of the time. Then connect the conductive part of the disk (Via a slip ring.) to a high input impedance amplifier. (The amplifier ground being connected to the zero voltage reference.) I think there would be an AC output if there was a voltage difference between the wires but no output if there was no voltage difference. This would probably need screening to make it reliable.
Les.