1967 Mustang wiring question?

I have a 1967 Mustang with a short circuit, and that's let me to discover a wire that I can't figure out: it's a red connector that powers the light in PRND floor shifter. The red plug in the harness under the dash has two wires connected to the same terminal: a blue/red and yellow/black. I don't see the yellow/black wire in any schematic in the Chilton's manual or the Mustang restoration handbook. It appears that this wire goes to a two-terminal plug behind the ashtray, possibly to the radio? The original radio was gone when we got the car 15 years ago. I don't know why it would have a different color when it's connected to the same OEM terminal as the blue/red wire. What was the function of this wire? At some point, the harness was cut into behind the gauges, and I have not yet removed the electrical tape to see what's under there, but I suspect that is where the short is occurring.