Assuming the gauge is getting the proper voltage from the Runtz, an "always empty" indication is caused by the other wire that connects to the sending unit on the gauge being at ground all the time.
Troubleshooting is fairly simple. With the key on, half a tank or so of gas and the gauge showing empty:
Remove the wire from the center of the gas tank sending unit. If the gauge now reads full, the sending unit in the gas tank is bad.
If the gauge still reads empty...
Remove the wire from the back of the gauge that the Runtz is not attached to. If the gauge now reads full the wire between the gauge and the sending unit is touching ground somewhere.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
| 
