Voltage issue

As Redfish posted, look for a big white connector in the underhood harness. The bulkhead is indeed one of the number one suspects.

Read this, even if you do not do the mod. This article describes what happens to the bulkhead, and ammeter. More rare but has happened, is the welded splice.

ANY terminal in the affected system can cause this........loose VR connector. Make CERTAIN the VR it tightly grounded. Brushes going bad (worn, greasy, sticky) in the alternator.

http://www.madelectrical.com/electricaltech/amp-gauges.shtml