You have a dirty fuel sending unit. To access it, remove the first aid kit box,(a couple of screws in the bottom). Then, disconnect the plug the attatches to the sending unit. Take a large socket, I think it's 44mm and unscrew the unit.
SLOWLY, withdraw the sending unit allowing fuel to drain completely. A couple of shop rags are a good idea to protect your seats.
Cleaning takes alot of care. there are some delicate wires that the float slides up and down on. There is a description of this procedure @ http://www.mbz.org.
But their removal procedure that called for removing the seats didn't prove to be correct for a 123 or a 126.