The speedometer needle in my E320 fluctuates up to about 30 mph. One mechanic suggested to replace the speedometer cable, another one the speedometer itself. Any experiences?

Also, the passenger visor's light is out, and on this one both agree that the whole visor must be replaced! Wondering if any of you has successfully managed to open it and change the bulb nevertheless...

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1991 W126 300SE (M103)
1995 W124 E320 Wagon (M104)