Hi I had the same problem on my CLk 430 1998, i replaced the 2 fuel sender on the tank.
You have to replace both because they work on series.
Senders are located beyind your car in the tank and you need to drain all the petrol before open the 2 senders, these because they are installed upside down.
To drain the petrol you nedd a special tool from MB because thesre is some system.....but if you open carefully once you can do this without too many mess.
Also to open the two senders you need a special tool but is very easy to built a home made....is just a piece of iron with 2 long bolt.
Sorry for my english
Alberto