'87 engine harness missing a circuit?

[Rob_85Ttype]

I have an '87 engine harness that I am installing into an '85 car. Both the '85 and '87 engine harnesses contain a 4-position female connector that plugs into a corresponding male connector near the HVAC blower motor. The '85 4-position male (1st pic) and female (2nd pic) connectors contain 3 wires - red, blue, and black. The '87 4-position female connetor (3rd pic) contains 2 wires - red and blue. Why doesn't the '87 harness contain the black wire? See attachments.

 

[Rob_85Ttype]

Anyone have an idea? Looking at some of Casper's harness pictures, it appears this connector is for the AC / heater feed. It looks as if his reproduction harnesses only have the blue and red circuits as well. I'm really curious about the additional black circuit in my '85.