If it is a re-pop stock or original harness it should already have the fusible links.

On my re-pop harness, there was poor connection at the spade terminal for the fusible link near the fender relay. It showed up after driving through rain as low voltage and cutting out with lights and wipers on. Inspection showed melted insulation at the connection. The link was still intact. I cleaned it up and installed a new link with soldered eye terminals and a #10 bolt to make the connection. No problems since.

Suggest you check every inch of wire from the alternator to the bulkhead and bulkhead back to the fender relay. Also pull the bulkhead connector itself and check for signs of heat.