1990 alternator wiring

I have a 1990 Mercury Grand Marquis (5.0 liter) and I replaced the water pump and intake manifold gasket. I had to remove several parts including the alternator. I installed the two wiring connectors on the alternator and discovered a third connector that was tied in with the two alternator connectors. I can only discribe the third plug as a single plug type connector. I can not locate where this plug goes and I am not sure if it is something extra. If anyone knows where this plug may go or what it may be I would great appreciate the information.