Several things happen over time to reduce illumination of the instrument cluster. One, the bulbs lose some intensity simply due to age; also, the cluster lens builds-up a film of oxidization on the inside; and thirdly, oxidization of the rheostat wiper introduces electrical resistance that effectively attenuates voltage to the bulbs even when the knob is turned full-on.
Solution for the first condition is to replace the bulbs; for the second, clean inside of the lens with a quality plastic cleaner and polish (such as Novus).
You have a couple of options with respect to the rheostat. If you think that you will ever want to lower the light level then you may want to replace the rheostat. However, most people find that W124 cluster illumination is just adequate at full-on -- and have no desire to reduce it further. In this case, save your money and solder a jumper across the wiper contact on the rheostat. This will give you the minimum resistance and maximum illumination (at least equal or better than a new rheostat).