I think I would start at the Ammeter connections on the instrument cluster. They were known to have problems over the years. Get a small mirror and look at the Ammeter posts on the instrument cluster, I suspect your going to find melted plastic and nasty looking terminals on the ammeter. You will need to disconnect the battery cable before you do anything more, because the melted ammeter + end will be battery voltage hot.

If the Ammeter is OK (I would be surprised), the next place to look would be the bulkhead connector. I would be even more surprised if you needed to look any farther. Gene