'87 Instrument Cluster - Please Help!

[4t5acp]

I have the instrument cluster ready to go back into my '87 V1500, but there's a particulaar wire that I can't figure out where it belongs. There's a bulb in a 1/4 turn twist-lock socket between the speedometer and fuel gauges. It is powered off the printed circuit. The socket also has a bracket on the side of it with a black wire coming off of it. This black wire has a round copper "plug" crimped onto the end of it. Try as I may I cannot figure out where this wire goes. The bulb lights up fine, as does everthing else - I think. Where does it go and what does it do?

Any help would be greatly appreciated - THANKS!