Electrical gremlins can surface in any car. Let's face facts. The more options you have and the more wiring running through your car, the more something is bound to creep up. Vintage Mustangs have about four or five wires coming off the ignition switch in the 1965-1966 years--maybe a few more as the vintage Mustang's power options grew. But none of the early Mustangs had wiring throughout the car as late-model Mustangs do. The ignition switch in your typical 1987-1993 Mustang has no less than 11 contacts, some of them with two wires each.
The design of the 1979-1993 ignition switch is simple. A pot metal section and a plastic or nylon section are held together with a few strategic crimps. Within these two halves is a sliding contact switch that makes contact from one wire to another or even to several wires at once, depending on where the switch is moved. The problem centers on those few crimps that hold the two halves together. With age and cycling of the switch, the crimps can weaken or even break, causing the switch halves to separate. This separation can cause any number of problems, from inoperative air conditioning to a car that won't start at all. Worst case is that the ignition switch can even cause a fire. Some model years were included in an ignition-switch recall from Ford, which concentrated on the fact that the switch terminals were too close together, which is an entirely different, and less common, problem.
The best thing you can do is not delay the repair another day and get that old hazardous switch out of your Mustang--which is exactly what we are going to do today with this 1987 Saleen hatchback.