Electrical short after re-installing instrument cluster

If everything except the amp gauge is disconnected the amp gauge or its wiring is the problem. The rest of inst' panel is not hot before ign' switch turned to on. So if short is present at switch off, again the amp gauge or its wiring has to be the problem.
Thanks for the responses. Yes, insulating washers are present. I have power into amp meter with Red wire, when I connect the Black I have sparks at battery post and no power to fuse block. There is no logical reason why a problem could have developed with the ammeter but I will replace it with an extra I have to check it. I did this exact same procedure on a friends 66 Dart with no issues so I am a little perplexed. I do feel it is in the ammeter though. Thanks again.