Actually most W123 cars have the same problem. The plastic housing around the instruments wasn't designed very well. I've had 4 or 5 of these cars & wasn't happy with the inst. lighting. There are 5 watt bulbs that will fit but could possibly melt the housing(one didn't). The W124 chassis had the same problem until 3/88 when it(plastic housing) was redesigned. A new housing will cure the problem. If the rheostat is in good condition.
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH