[7up]

I have a 72 gmc pickup and I can't find the original or a piece of the original wire that ran to the sending unit.so what I'm trying to do is to use the gauge itself and hard wire it,but the problem I am having is everything because I'm not understanding what order all the wires should be in from power to ground,also the fuel tank for my truck is not the original tank it was replaced with a tank from a 97 isuzu rodeo,and on the fuel gauge it has two terminals.

[Stratmando]

The gauge gets positive voltage when ignition switch is on. The other terminal on the gauge goes back to the sender, the other sender terminal goes to ground.
The sender provides a variable resistance to ground.

[KISS]

Most fuel gages are nothing but a variable resistor. As the tank fills, the resistance changes.

There is a micro/milliameter in series with the fuel gauge which records the level. The fuel gage is oil filled so it doesn't show the sloshing in the tank.

Now the tricky part. There is a fuel gauge regulator. It generally pulses the series combination of the sender and meter at a low frequency and lower voltage.

Pulses are used to keep the power dissapation very small

They are not all the same.

Certain tanks have a thermister embedded in them to detect low fuel.

They are not that simple.

Sender resistance is usually in the service manuals. The regulaor specs usually are not and neither is the meter full scale current.