Hi, if the speedometer is out the problem is most likely the speedometer itself. There is not a separate fuse for this problem.
The car will need to be connected to the Mercedes tester to check to see if the speed signal is reaching the speedometer.
If the speed signal is present they can try resetting the instrument cluster but it may not help.
Ultimately the instrument cluster may require replacement to repair this fault.
Let me know if you have any questions.