Just as a point of reference:
My SLK32 AMG had a fuel gauge that was not working at all. I ordered a fuel sender unit assuming that was the problem. After replacing the unit, the gauge still did not work. On the advice of another thread I checked the connections to the gauge cluster by removing the fuse panel access on the drivers side and the vent. Using a flashlight and my hand I was able to check the connections on the back of the gauge cluster. The one on the left(drivers side) was not tight. I tightened it up and locked it back down and the fuel gauge came to life.
The morale of the story - check your gauge cluster connections before ordering a fuel sender !