the harnesses don't normally go bad, if you think the connection at the sending unit is bad then I would unplug the connector and use a jumper wire to connect the harness to the sending unit ( color to color ) or with the harness disconnected look to see or ohm out the harness for continuity between the connections in the plug basically shorting the sender out of the circuit and giving you a full reading on the gauge. what test did you do to see if the sender was ****? if the variable resistance is actually in the sender, like it should be, then you need to check the resistance at full and empty levels and make sure there is a difference.