matt
the switch is part of the light assembly in the top of the glovebox houosing - sounds like the contacts for thh switch are dirty/corroded.
If you remove it and clean it up it should be fine.
To get it out you need a small stubby flat blade screwdriver, prise the bulb holder/switch assembly out of the surround, disconnect the wiring plug and give it a good clean - refitting is a reversal of removal