I don't know what the wire was for originally, but like longhornmail said, if it has the full 12 volts (with the key in the "start" and "run" positions) it should work great!
If is is powered correctly, start pulling fuses with the key in "run" to see which one provides the power. This may give you some more clues.
Good Luck!
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'56 F-100 (w/all GM stuff)
Lookin' for another parts chaser! 
'05 GMC crew cab
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