Welcome to the Stovebolt RedTruck!
Plastic covered wire has an expected life of about 20 years. Yours is 40 years old. While it might be possible to patch it up and keep it working it may be less demanding to replace it.
I'm sure someone will tell you that repairing the old wire can be done. It often can. But if you are unsure of your skills with electricty you may find it far less trouble to simply replace the harness.
