Do you have a wiring diagram?
FIRST Do you have the ammeter hooked up? These are the big red and black wires under the dash, the only two with ring type wire ends. These must either be hooked to the ammeter or bolted together and taped.
Here's a simplified diagram of the main power feed from Mad Electrical:
Your main battery feed is from the battery, to the star relay, and from there a fuse link that GOES THROUGH the bulkhead connector, so that is one of the first things to check--see if it's getting both sides of the bulkhead
From there the red wire feeds to the ammeter
Coming off the ammeter, notice the big black goes to a SPLICE. This is a big factory "welded" splice, taped up in the harness under the dash. IN my lifetime I've seen AT LEAST 4 of these fail.
From the splice, power feeds off to the "hot buss" of the fuse box, the headlight switch, and the ignition switch.
Then this BLACK large gauge wire continues on OUT the bulkhead connector, and ends up at the alternator. Under normal conditions, if this circuit is intact, the alternator output stud will be "hot" with 12V
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
