You can download or view the full parts manual v5.2
here. It includes wiring loom layouts which although not very high definition do give you some indication of cable routings. Doubt it's a fuse issue if tacho illuminates. You could try connector to speedo though if you can gain access.
ETA: I have a vague recollection of similar issue with illumination. Not sure if you are aware but it's adjustable although not mentioned in manual. With ignition on press end of indicator stalk to cycle through clock, trip etc. until voltage comes up. Then press and hold until
ILL comes up. Then pressing it quickly allows you to cycle through illumination levels 1..10. Have heard that it sometimes affects only one instrument - worth a try.
