Unfortunately 1998 was the year MMC changed wiring loom suppliers for plus 8, so it's difficult to know whether you have early or late version, see
here.
If later type then the illuminated switch was for rear fog light only feeding power to it via fog light control module to ensure it only worked with headlights on. The switch LED illumination had two feeds fed by individual diodes, one of which was common to other illuminated switches, so maybe night time illumination whether switch was on or off. This would explain the effect the dimmer had.
