Last I knew the gauge light fused was tied into the running lights, I would start by checking your bulbs for clear contact, making sure they are the correct bulbs, get an ohm meter and check for shorts, or bare wires contacting ground (with battery disconnected), start with reverse circuit, something is drawing the amps and rocking your fuse ( try disconnecting your reverse pickup wire on trans and put the truck in reverse see what happens)
Good Luck
