Please tell me what this wire does?

I've been working on the dashboard lights on my 72 Mustang HT. The headlight switch probably needs to be replaced, but I found this wire dangling next to the switch. Colorcode is Black-Lightblue. I can't find where it plugs in. In the wiring diagram it is connected as follows: Map lamp - R.H. Front door jams switch - R.H. Instr. panel lamp (logical, but none of the dash lights work) - Headlamp switch - L.H. front door jams switch - Dome lamp. So please help me locate where this plugs in, it might clear something up. Here's the pic:

It is for the optional underdash courtesy lamps.

you will also find another connector in that area similar to it that is for the optional Parking brake warning light. there are some connectors not used on cars because the harness had the wiring for optional accessories that are not used on your car.

kind of annoying but on 71s adding a dash map light is easy.

starting in 72 ford moved the connector for the dash map light onto a daughter wiring harness thus if your car did not come with map light from the factory it is a hack job to install one on a 72,, yet they left the courtesy lamps wiring connector alone..... so you can have courtesy lamps but not easy map light... where on a 71 you just plugged stuff in no problem.