The good news is that the on-and-off headlight cycling provides you with some measure of intermittent nighttime visibility. The bad news is that unless you replace your headlight switch soon, you'll find yourself driving down a dark highway with no headlights at all, putting your treasured vintage Mustang at risk. But there's more good news because it's relatively easy to replace an aging or faulty headlight switch, especially if you understand the procedure.
As Senior Editor Jim Smart pointed out in his informative "How to Troubleshoot and Service Headlight Switches" article in the June '10 issue, '65-'73 Mustang headlight switches are prone to corrosion as they age, which takes a toll on the switch contacts as heat builds through high resistance. Headlights are high-load electrical components, so they draw a lot of power, causing the switch to get warm. With new switches, the heat isn't a problem because the circuit breaker and contacts are clean with good continuity. But as switches get older, the heat causes the circuit breaker to do its intended job--cycle the headlights off and on. More powerful aftermarket headlights draw even more power than original-style headlights, exacerbating the problem.
If you experience flashing headlights, then it's time to replace your headlight switch. Thankfully, the procedure is fairly basic for vintage Mustangs. Here, Merv Rego at Classic Creations of Central Florida is replacing a faulty switch in a '66 Mustang, but the R&R is similar for other Mustangs.