At a guess, if you've checked fuses inside and under the bonnet and looked for any loose earths bolted in the footwells (they are on 80s so this is a guess) then I believe there is a voltage regulator somewhere that controls the voltage to the instruments. This may be at fault. Couldn't tell you where to look though but under the dash or at the sides of the footwells are likely spots.
Good luck with it.
Oh, and welcome BTW. This section is really just for saying Hi and telling us a bit about your cruiser. You may get more help by posting your question in the 105 series section.