Doesn't look that bad. You could replace the harness. I would heat shrink repair the wire and check/clean the connections. However take it a step further. One you can do the Nacho suggestion and run a parallel wire from the alt, through ammeter and back to the battery. Second you could run a larger wire from the alternator around to the starter solenoid, effectively bypassing the ammeter. Current takes the path of least resistance. This will cause the ammeter not to work properly, in which case you'll want to convert to a voltage meter. There are a couple how to's on the board to remove the ammeter from the rallye gauges and insert a Sun or Bosch voltage gauge into the cluster. It looks almost stock.