Headlights, brakelights, heater, and gauges all dead?

With no headlights, I'm surprised you have anything else. Sounds to me like you need to disconnect the bulkhead connector and inspect it. If that doesn't show anything, pull the cluster out if necessary, and check over the ammeter wire terminals and the ammeter connections. While you are "up in" there I would check the black wire from the ammeter, follow it down a few inches and check the splice. You will have to cut open the harness for a few inches.