The two together are for the speaker. I think the gray/red wire is for the radio lamp, and the black wire is the power feed, but I'm not positive. I know that Ford used a light blue/red wire to supply voltage for the instrument lights, and other lights controlled in that circuit, for years and years, and that the power feed from the ignition switch was often black/dark green. If it's possible that your black wire is actually black/dark green, then I'm sure of my above suggestion.
Take care
~Chris
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RACECAR spelled backwards is RACECAR
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