If the metal clip is electrically hooked to the wire, I would suspect it to be a ground and the clip does not plug in anywhere, but rather the clip /slips onto the radio chassis somewhere. I have seen grounds like that..they don't plug in , they connect mechanically by clipping on.
Just a thought.

Ohm it to ground and if you get trace , it's a ground wire to somewhere/something... possibly ash tray frame ?????

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A Dalton