I want to replace the fuel level sender on a 1991 420SEL but I don't know how to locate it.

Pull out the back seat. On the left hand side behind where the head rest would be is a black plastic cap, 2" in diameter. Pull off the cap and you will see a big nut with an electrical plug connected to it. Thats it. You will need a large socket to take out. I can't remember what size it is 28-30mm. Do a search and you will likely get better info.