Hi Chris,
Yes, that is a known problem with the stock Veglia ammeter. There is a write up on Mike Dailey's Pantera Place website re the "stacking order" for the nuts, washers, and wires. But, if you have a bad insulator, that will need to be replaced.
http://panteraplace.com/page178.htm
Some owners have changed to a volt meter. If you do replace the ammeter with a volt meter, make sure that the wire that feeds the volt meter is switched. Otherwise, the volt meter will be drawing current all of the time (even when the ignition switch is off).
If you are not trying to keep the original look of the gauges but want an ammeter, there are several good brands available.
John
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.