We can not tell you what wire you are talking about. bd gave you the links to the wiring diagrams, if you could find out which black wire you are talking about and reference the coded number associated with it, we can tell you what it is for if you are not able to follow the wiring diagram.
Like they say, a pic is worth a thousand words. Especially if you can show a clear point of the fuse block where the wire comes from.
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