amp gauge

That shunt looks awfully short.  The purpose of the shunt is to provide a voltage drop.  Thus being an 80 amp bar you may need to file a few notches in it order to give it less current capacity.  At one point you had said the meter was for the test function. I just assumed it was a stand alone tester.   No worries the circuitry is still the same for this.  Does it charge a battery?   Use your meter and monitor the voltage at the battery.  Do not leave it unattended.  Just read battery voltage and then charge it a few minutes and unhook and check it again.   There is a good chance that the volt meter is acting crazy because of the burnt potentiometer that controls it.  Have you ever seen a fuse element that looks like a lightning bolt?  That is the effect we are looking for on the shunt.  It may need to be longer than it is.  The longer it is the more voltage drop you can get across it.  The amp meter is just measuring the voltage drop across the shunt.  A little current, small voltage drop.  A lot of current a lot of voltage drop.

If the wires that go out the back are the wires that connect to the battery, the meter negative needs to be connected to the end of the shunt that is nearest the camera.

Since you have the 80 amp bus bar you may need to make it look like this fuse.   The thinner areas of metal will allow voltage drop.  With it being only 2 inches long and having the 80 amp capacity 40 amps of current will not be enough to get any voltage drop.