Hello llong,
That wire attatches to a small pin/post that sticks out the backside of the drivers side alternator.
If you have an electric tach, that post/pin is where you would connect it to, to get a signal.
I remember that wire, because I had to replace both my alternators a few months ago,
with military alternators, and the Drivers side alternator had that post/pin.
I was also told that wire goes to the diagnostic port... So it can be left disconnected,
and your charging system should run fine anyway.
Good luck, good night, God bless you...
P.s. I too will install a tach, perhaps one similar to this so it blends in:
http://www.alligatorperformance.com/userfiles/image/aut2305.bmp
But I too, will have to wait until money grows on trees or pennies fall from heaven.