I am just crafting an article on exactly this topic, as both the lights on my 1984 Cabriolet and my 1990 LHD Convertible have appalling instrument cluster lights.
I have greatly improved the lights on the Convertible by taking the instrument cluster to pieces, removed the speedo and rev counter gauge dial fronts. Then I proceed to remove each of the little green domes (which cover each instrument light) – then put it all back together. I was actually taking the instrument cluster to pieces for another reason, which was to change the KPH speed face for a twin KPH/MPH dial face.
This has made a reasonable improvement to the backlighting of the dials – in fact is probably good enough (yet to do the same on the Cabriolet).
I imagine that that with the little green domes removed then higher watt bulbs can be used, as there is then nothing in the way to melt.
I have also looked into replacing the usual filament bulbs with led versions – which I’m yet fully investigate.
I do need to get on and write that article now, as it includes photos (I have them somewhere) of each dismantling stage).