Use this diagram from MAD electrical to help understand the major supply path. The power comes into the interior of the car through the
fuse link, and goes next to the
bulkhead connector
From there it goes through the ammeter, (red wire) and then comes out (black wire)
The MAIN supply for the interior, if the path is OK up to now, is the IN HARNESS SPLICE, sometimes called the "no1 splice" This is a factory welded splice in the ammeter black wire, taped up in the under--dash harness and supplies
the headlight power to headlight switch (but not tail, park, or dome)
the ignition switch incoming power
the fuse panel hot buss
maybe a couple of other things in some cars
SO get a test light or multimeter, and identify the fuse link which normally "looks funny" and usually has a rubber "tag" on. It comes off the starter relay big stud and goes to the firewall connector. If you have power there, move to the ammeter and see if IT has power.
If not, it's either a bad fuse link, or trouble in the bulkhead connector.
If you have power on both sides of the ammeter, turn on the headlights and see if power drops off. (Putting a load on helps show up a poor connection)
If you still have power on both sides of the ammeter, unhook the battery, do what's necessary to get into the dash harness, and untape, following the black ammeter lead. These splices don't fail very often but they can, have, and do.
Still one more check, with everything hooked up, and headlight switch on, is to see if you have power at the alternator output stud. If you do, but have no headlights, it's a bad factory splice.
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
