What is this wire for???
No mention of what year Mustang; the wiring changed several times between 1979 and 1995.
No mention of what engine type - 4-6-8 cylinder engines all used different engine wiring harnesses
No mention on what harness you are working with for each connector/wire set. There are different harnesses for engine mounted EFI, main computer harness, under dash harness, front body harness, rear body harness.
When you post electrical questions, you need to do two things:
1.) Include the wire stripe and colors, like blue/red. First color is the solid color and the second color is the stripe. Color photos don't work well, since each computer reproduces colors differently.
2.) Always show the face of the connector where it plugs into its mating component
Answer all the questions and I will try to help.