Ammeter waving when ignition is on.

The causes of this are "in no order"
Bad connections in the harness and grounds
VR if it is the old electro-mechanical type
Sometimes the battery but very rare

You need to go clear through the heavy ammeter/ charging circuit. Refer to the diagram in the MAD article. The connections at battery, starter relay, fuse link, bulkhead connector, ammeter, possibly (but rare) the welded splice), The VR field connections and the path back to the key and through the ignition switch and to it's sourse including CONNECTORS (terminals) and the VR GROUND.