I'm going to say that's the clutch safety/neutral position/starter relay trigger wire, going off the colours you mentioned, but you
really need to put your truck information in your signature so we can stop guessing.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.