I usually open up the cluster, take out all the guages and clean the cluster with soap and water to take out the film of dust. You could also take out the celar palstic which transmits the lights to the front of the cluster. TO do this, have a soldering iron ready to melt about 4 plastic pegs. To replace, just glue them back. Worked for me. My 77 300D works better. I also hot wired the rheostat so it would always be on full.
Hope this helps!