The ash tray light would come off a gray wire and the bulb would ground to the ash tray. The rear defroster is part of the switch and is a diode not a bulb.
It's a stiff black wire with a bulb at the end of it...right? That has always stumped me. I plug it in to the top right slot on the passenger side of the cluster. It doesn't show up on the wiring diagram and it makes no sense to me. Was your car made in Van Nuys? The wire isn't fiberoptic, I think it's just stiff. It's probably a ground wire. You can easily check if it's ground with your ohm meter.
I've just checked two spare harnesses I have in my garage, one from 77 and one from 78, neither have that bulb. But two cars I have had, built in Van Nuys, have had that screwy wire/bulb.
That's something I'll have to research once I tear my car apart again. I guess it could be something outside the harness altogether.