Gas gauge/Sensor

[BnObob]

I have a problem with the fuel meter on a 1989 300E sedan. At about 1/2 full,
the needle jumps: E, F, wherever. When it goes below 1/2, it settles down.
[I had a similar problem in an old 115 body 240D, and lifted the trunk mat,
unscrewed the sender, disassembled, cleaned the gunk, the nichrome slide
wires, and cleared the problem.] But on this 300E, where the tank is the support for the rear seat, I cannot find access to the tank, nor sensor.

I suspect access is through the trunk. I have only the Chilton books, not
a real Mercedes CD/manual. Can anyone tell me if the sensor is the suspect,
and how to locate/remove the assembly?

Thanks.....