jisaac,

If you have a VOM with a diode checking function, measuring tip to sleeve on either of the 3.5mm plugs should give a reading of about 2.4V in the forward direction. This is assuming about 1.7V for the LED plus about 0.6V for the series isolation diode. Next, check from 3.5mm tip to tip. There should be no diode conductivity here.

I'm not sure what the resistance will be when measuring between the 3.5mm "sleeves" because XANTECH may have inserted series resistors here. Certainly, there should not be an open circuit.

Finally, measure the voltage between the two source chassis while the 3.5mm plugs are not inserted. Measure both AC and DC potentials. This should be very close to zero. If this potential is not close to zero, try adding a chassis to chassis bond wire.