Read this article, even if you do not do the bypass:
http://www.madelectrical.com/electricaltech/amp-gauges.shtml
For this thread, the meat of this article "could be" the diagram they posted down the page, which represents the simplified diagram for years and years of "these girls"
Now, follow along on the diagram. From battery positive, juice flows through the fuse link, the bulkhead connector, and to and through the ammeter. Then juice goes down the BLACK ammeter wire to the WELDED SPLICE
Notice that it goes off to the LIGHT SWITCH.
There IS NO fuse of any kind for the headlights except for the fuse link
EDIT I should have said before the switch---the switch has a built in breaker
This splice also branches off to the hot buss in the fuse panel, and the main feed to the ignition switch, and a couple of other things (wiper switch in some years)
So what could be the trouble?
Any of the mentioned main connections. The fuse link -- the bulkhead (red) connector terminal, the ammeter terminals, the ammeter, or the welded harness splice
Probably the "wiggle test" is a good place to start. Turn on the lights, the radio, the heater blower, and wiggle the bulkhead connector. Reach up and feel for the ammeter and wiggle the wires there.
If that does not show up anything time to reach for the multimeter. We can detail some more tests.
It is rare, but that welded splice DOES and HAS failed. It's taped up in the under- dash harness down from the ammeter black wire a few inches. Save it for last -- if you eliminate other stuff, time to pull the cluster, untape the harness and inspect the splice.
The first bad welded splice I found, my bud's could not believe it. This was in the parking lot of my (sailor) bud's apartment complex in about 1972 or so. His 69 RR had failed that splice. We had the cluster and half the dash wiring spread out all over the seat. I know (and he said) "are you sure you know what you're doing?"
