take the whole boost gauge out. there is a little light on the circuit board that is in a holder that twists in. its tiny, it possibly may be the same bulb and holder for hazard button light, rear demister button light and air control. i definitely know its the bulb and holder for s13 air control unit, demister button light etc.
light could be blown or just may have fell out.
i just fixed mine, it fell out, had to twist it back it and all good now