One thing to never forget - the wiring in your muscle car is forty years old and updating to modern and safe components is easy. Owners and restorers of 1964 to 1966 first generation Mustangs can take comfort in the fact that there is now an electrical system upgrade from Painless to completely rewire this classic car. These Painless wiring harnesses feature many pre-wired connections and are color coded to make installation a snap.
1964-1966 Mustang Wiring Instructions (PDF)
Note: Some firewall modifications required.