Katana Owners Club U.K.

The bulb in there is a cap less very small bulb, I have replaced them with bulbs from Maplins.
Take off the unit ,take cover off. Find bulb required and at the bottom of the unit will be2wires bent over from that bulb straighten them and it pulls out, put new bulb in and bend the wires from new bulb in to place.

make sure its getting a signal from the sender unit first though.

To be honest if only 1 light aint working it is normally the bulb as the bank is live and each gear selects the earth, only the neutral from the switch by the gear lever is seperate , ( Blue one )
However to be sure as john says swap a working bulb to be sure.