Don't Cut It, Plug It!
When you need power for accessories, lights, and the like, you don't have to cut and splice connections. Ford provided female bullet plugs throughout our Mustang wiring harnesses for instrument lights, sound systems, cruise control, and more. Use these plugs for light-load accessories only. Heavy loads, like high-power sound systems and power windows, need an auxiliary electrical system with its own protection. This is a three-hole plug for instrument lights, including the radio and Rally-Pac. There are other plugs for courtesy lights and accessories. Remember, do not cut the harness when you don't have to.
A Switch In Time
While you're at it, replace the switches. Headlamp switches deteriorate with age, causing the circuit breaker to cycle your headlights off and on. Contacts in switches become pitted and useless with time. Corroded contacts create increased resistance, causing heat, and possibly starting a fire. It's also a good idea to replace all of the light bulbs, including headlamps.