Well,
now that I am becoming familliar with the signal tracer (Klein Tools VDV500-820), I have traced the red wire in its confusing bundle of conductors of various hues under the driver's floor mat, to include removing the plastic "raceway" cover over the two under-floor wire bundles.

I also briefly tagged the "singing" wire in the area under the dash, just above the drive's left foot.

There is still no sign of a "loose end" as per the manual, so I susspect that the wire might be terminated on an unused socket. Hey - its always good to have a theory - at least until said theory gets dashed to pieces on the rocks of reality.

The search will halt for today when "Miller Time" rolls around.

Your post is old and I don't come here a lot ----- BUT the wire is in a bundle behind the kick panel to the left of the driver foot. It does not terminate anywhere. Keep pulling on the individual wires until you find the one that goes no where and comes free from the bundle.