alternator, 4th wire?

[Matt Sfeir]

Just finished (almost) replacing the alternator.  There are 4 wires coming out
of the alternator wiring harness:
1. bolts on stud with 12mm nut
2. large electrical connector
3. ground attached with small screw
4. small electrical connector attaches to?

I don't know if it was dangling there before or not.  I've tried looking in
Haynes, Chilton, and FSM but couldn't find any good pictures.  Does anyone
know where that 4th wire goes?