there is no ground needed on the stock console "ammeter". It is really a -0.5 to +0.5 range millivoltmeter labeled in Amps. It uses the resitatnce of the red charging wire that runs between the horn relay and the battery to provide the voltage drop to deflect the needle. When the battery voltage is higher than the horn relay terminal voltage the needle deflects down scale towards discharge, when the horn relay volatge is higher than the battery voltage the needle deflects upscale towards charge. thats all there is to it.
If you tried to run anykind of current thru it, like grounding either end of the black, or black with white trace wire, while the other is still connected, you ran 12V thru a 1V range meter and probably smoked it.
It doens't take alot of current to burn up the wiring in the gauge itself. If you've replaced the red wire that runs across the radiator support from the horn relay to the battery terminal with a large size wire the gauge may not even register the difference and will sit at the 0 point all the time because the voltage difference across that wire for normal charge or discharge currents is no longer high enough. Do not under any circumstances try to run current thru the gauge, or attach anything to the circuit higher than 1V DC, you will be buying a new gauge if you do that.
Measure battery voltages at the console connector on the black and black/w wires. You should see battery voltage on both, if not the wires are open somewhere.