I believe there was a change in the dash cluster lighting for the 124 about 1988. As far as I know the change did not increase the number of lights (2) which illuminate the cluster face. Higher wattage bulbs have been tried but these will melt the plastic over time. In fact, the plastic around the bulbs in my '86 300E was a bit distorted and darkened when I pulled the cluster to have a look at the lighting which was too dim to see while driving at night. I could barely see the top of the speedometer and the gas gauge.
I ended up removing the two dash lighting bulbs and the prism system. The prism system was supposed to capture the light from the bulbs, transmit the light to a reflective 'mirror' at the top, front of the cluster, and then reflect the light down onto the dash instruments. This system is very inefficient.
I placed 4 LEDs at the place where the prism mirror was located. I roughed up the surface of each LED with steel wool so the the LEDs would send out a diffused light rather than the usual sharp beam. The LEDs get power from the 12v input to the dimmer control. I used a separate dropping resistor for each LED, It was either 270 or 470 ohms, I don't recall now.
I painted the area behind the LEDs with white model paint to help reflect more light down on the instruments.
As a result, I can now see all the dash cluster instruments very clearly. There is no dimmer function for the dash lights but the dimmer still controls all the othe lights connected to it.
