To memory there should be two bulbs somewhat tenuously connected along the centre line, i.e. behind the left and right dials. If the bulbs are missing it could be you have to dig around for the scraggy bits of wiring they plug into.
I had to replace the lamp holders because the old ones were melted by the heat. And I inserted LEDs. I completely disassembled the instrument panel to check the fan potentiometer, a defect of many.
Thanks everyone, I've had a look again and the reason I couldn't find the bulb was because there wasn't one! I bought some 5W push in bulbs and put them in what I assume are the bulb holders but no joy. Does anyone know what type the original bulbs are?
Yes, they are T5. My leds are with separate polarity, a lot of work and tests when I replaced them on the Tacho. It could be the defective lamp holders, with the heat the dietal tabs move no longer making contact. These are the lamp holders that I used. You just have to fix them somehow.