I heard or read somewhere that it could be the connection to the lights in the back - but I have had the tail lights disconnected and I don't remember the fuel gauge acting weird in any way.
The sender in the tank is actually pretty easy to get to - with the right tool.
Remove the fabric/carpetting on the floor of the trunk, and then the one that is against the front of the trunk cavity (opposite the back seats). This will expose the fuel tank. You will then notice the fuel sensor plug and a large hex-bold-type head thingy.
Unplug the sensor connector (its a 3 or 4 pin plug which only fits one way). Might be worth checking it for any corrosion or bad solder connections.
The biggest part of this is undoing the hex screw. Its a very large size. We had to get a special socket from ackland-granger - as most other hardware shops did not stock sockets that big (btw. this same socket is the right size for the block heater - if you have one for cold environments and it burns out).
It does not take much force to undo it, once you take it out you will find 2 O-rings (On the hex screw) and the white top of the fuel sensor. The fuel sensor assembly pulls out easily (just up and out) - there are no other clips or screws or bolts holding it). The hex screw just rotates around the top of the sensor assembly, the sensor assembly is not directly connected to that hex screw.
Make sure nothing falls into the tank while you are working on the sensor. I couldn't figure out how to take the sensor appart, but for my fix I didn't need to - there were enough holes in the sensor to get the gas in and let it drain back out.
Put back in, careful to not over-tighten the hex screw (I couldn't find a torque setting - it really doesn't need much). Plug the connector back on and thats it!
hope this solves your problem.