could be the sender or the guage, i dont know how easy it is to get at the sender but may be worth taking it out and having a look, sometimes they can get stuck or the float fills with fuel causing problems, does the fuel gauge allways stay on half?, does it stay there with the egnition off 2? because it stuck half way i wouldnt really expect a wiring problem.i would say most prob sender but not allways the case. hear the sender built into pump but dunno if this is case with every model, isnt the access holes under the rear seats, worth having a look
try this link and press play button it shows the operation and how they work, may help ya
http://auto.howstuffworks.com/fuel-gauge1.htm