I have this knob right behind my auto gear stick, between the cigarette lighter and the alternate radio switch. Have no ideia what it's for. It turns about a full circle.
Thought it might be a volume knob or speaker audio balance but tested it and nothing. Anyone with a similar artefact?

Car is an XJ6 Series 1.

 

That would be something a previous owner has fitted.
Easiest way to find out would be trace the wiring to it's destination.