Take a look at the gauge cluster schematic dmjlambert made on this thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=709847
Power for the fuel gauge is provided on pin 3 of the 12-pin connector that plugs into the back of the gauge cluster. Check for 12V on pin 3 as well as the post on the back of the fuel gauge that is connected to pin 3 via the circuit board. Also check for continuity between pin 4 and the other post of the fuel gauge as well as between pin 4 and the terminal on the fuse panel where the tan fuel sender wire plugs in.
There are variations in how the side marker lights are wired between the different years of the 67-72 trucks.
http://67-72chevytrucks.com/vboard/s...=707545&page=2