What is this wire?

What is this push connect wire go to?

This is where the main harness splits out by the starter relay (now relocated in the fender)

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Here's the stock location, you can see from the other photo it sticks out about 1" after the chassis harness splits off into the spirder web of wires.

You can't see the little connector in this picture, but this is the location:
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I don't have a stock a/c unit (never has the 5.0 stock a/c)

so...i can tell you that the a/c plug is 2 wires into a farily good size plug, this mystery wire is not part of that....unless you're saying there's a 3rd wire that goes in there?

I think I seen the original cover to this myster wire is a small skinny gray type. (very small push connect style)
 
There is a small open-ended gray connector in that location with two B/W wires that is used to test the windshield washer pump. You can verify by sticking a piece of wire into the connector and touching the other end to the positive battery terminal. Put a rag over the nozzles and listen to the pump.