Most of the control panels in the cockpit should have these (unless someone has carelessly painted over them).
Behind the "+" is the power and earth connection for the panel lightplate. Most panels are made of clear plastic (with a layer of brown paint in the case of Boeings). Bulbs are located at various positions in the plastic and the clear plastic allows the transmission/refraction of light across the whole panel. The light comes through the panel where the lettering is.
Pictures are provided in the many previous postings on this subject.
Rgds.
NSEU