Here is how to get the seat out or get it from the trunk...
Take the bottom of the rear seat out. Look about a foot in from the edge of the seat, at the seam where it meets the floor. You will see a small lever, pull it towards the door of the car, and the seat catch will release. You can then lift out the rear seat bottom.
Carefully remove it and set it aside.
Next, look along the bottom of the rear seat back. You will see three bolts (8mm socket here), with big locknuts on them. One will be on each edge, and will bolt into the side of the car, the third one in in the center of the seat bottom, but may be concealed by a black plastic piece designed to retain tension on the seat belt buckles.
That piece just clips on to the two metal brackets, that extend up to the seat belt buckles. You can easily remove it (you will understand once you take a look at it.
Once the third screw is out. Push the seat up from the bottom. It has to come up around an inch and a half to clear the clips. Once its up, pull it out toward you. You will have to slide the seat belts off the edge of the seat, but after that it will come out easily.
The fuel sender / fuel tank may be accessible there, but I doubt it...
Personally I would go in via the trunk.
When you look at the trunk you will see what looks like a carpeted back wall.
Look closely at where it meets the floor and the rear deck. You may have to pull the floor carpet back slightly, but you will see six 8mm nuts, brass in color. Three across the top, and another three across the bottom.
Remove those, and that back "wall" of the trunk is free to be removed from the car.
This will give you full access to the fuel tank....
That wall is basically designed so you dont have things rolling around and banging into the fuel tank.
As far as the fuel sender - take a VOM meter and measure the ohms it is giving off. It should be steady for any given level of fuel in the tank. You may be able to change the reading slightly by parking the car on an incline, to confirm proper operation.
If its giving a random reading i.e. resistance jumping all over the place then the sender is in fact bad.
Other things that come to mind are possibly a loose ground - does the gas guage dance in motion? Or when the car is stopped as well?
Or possibly a faulty gauge..... This will confirm if the actual sender unit is bad or not.
Hope this helps,
1990 500sl R129 - 76k Original Miles - New project - Follow the saga http://90r129.blogspot.com/
1990 190E 2.6 148k mi (sold)
1989 420 SEL 246k mi (sold)
1995 C220 175k mi (sold)
1992 190e 2.6 74k original miles (sold)
2000 c230 Kompressor 122k miles (RIP)
1996 C220 149k mi (sold)
2000 C230 Kompressor Sport 127k (sold)
2009 Mercedes c300 4matic
2006 Mercedes s430
2005 Jaguar XJR
2003 Cadillac Escalade