"It's where the panel is earthed to the airframe,"
It's both power AND earth. The connection is a coaxial type arrangement, like you see on on some TV antenna cables.
If I remember correctly, there is a power pin in the middle and a circular earth (which may be divided into quadrants to allow expansion)... of course with a gap/insulation seperating them.
The control panel faceplates are usually painted clear plastic/perspex. On the back of the perspex is a printed circuit board with miniature lightbulbs attached to it. There is a single source of power for the circuit boards (i.e. behind the "+"). The clear perspex spreads out the light across the whole panel (refraction), so you don't need too many bulbs.