mystery bulb and wire - where does it go

[full foot notch]

so while trying to tidy up the wiring on the truck, ive came across this bulb and wire behind the instrument panel, ive tried tracing the wire and it goes into the big bundle of wires in the loom, I also tried looking on the wiring schematic that's pinned on this site, but I for the life of me cannot figure what it is used for and how to test to see if it turns off and on as it should from factory.

the wire looks to be gray/silver and the black you see on there is overspray from previous owner. my first thought was that it might goto the instrument cluster as a warning bulb, but all the holes avail are already taken and non of them correspond to that type of socket holder on the bulb.