Fuel Gauge
Most likely its the float, the old design has corks in the ends and the float usually sinks to the bottom of the baffel, or the magnets have come loose.
Rather than me explaining the whole operation go to xxxxxxxxxx.com he has the part and has a pictorial of how to replace it.
It also could be the fuse thats located inside the baffel, but I would try the float first.  I had the same problem and it turned out to be the float.
Lou