Where the bulb resides
Tried to send you an email but it was returned. The diff left Friday as UPS screwed up on the pickup. The tracking number for it is 1Z210 X4503 3907 1564. It should arrive Mon or Tues. You original speedo will be off by around 10% and don't know if MB uses a gear on the output shaft or uses different speedos for different ratios. MB Doc would know for sure. If you need a 3.42 speedo, mine with 138k showing can be yours for $50 as I went to a digital unit which also gives 0-60 and 1/4 mile times.
The left side bulb resides in the HVAC pushbutton unit and it has to come out to replace it. After pulling the unit there is a slim plastic panel which has both bulbs on it. Unclip the panel..carefully..and raise it to see the bulbs. If doing one, do both considering the age and trouble to replace.