Disassemble your gauge pod with it still connected(you don't even have to take it out of the dash). Take the trim off, then the clear protector, then the black trim, and the section with the volt meter should pull out(may have to lift another section a little). You will see where the two prongs of the volt meter plugs into the case. Turn the key on, and probe these with you test meter, if you have 12 volts, your gauge is bad. If you don't...then you have a wiring issue or the Trace Board on the case is bad, or you could just have a bad bulb(some of the bulbs on the case route ground to other areas of the Trace Board. If it's the Trace Board This is a easy fix if your handy with a soldering iron. A bulb is even easier. If it's the gauge it self, it's BROKE.
Ford wanted $167 for mine(Volt & Temp on same panel, both broke). So I called AUTOMETER, and ordered the 1" electrical gauges, drilled my existing panel out, and wired them to the case, installed their sending unit for the temp, No wires to run, no tubes to break. It looks great too. I am planning on doing the whole cluster in the future, but will have to make a custom panel first, since AUTOMETER doesn't make a 1" fuel gauge for our sending units, only 2".
Hope this helps.