yep, thats a inline fuse holder, most likely for the radio, or someone added it for another circuit. follow it to its source, i bet its just a power lead for a radio or other audio device.
like i said in our pm, the fuel/volt gauge;s light bulb is usually a metal pronged fitting that fits into the hole on the backside of the gauge housing & the heat a/c bulb wire has a black plastic fitting that presses into the opening.