headlight wiring diagram 85-1
Trinty_gt, you do have some good points.
If it's ok, can I expand?
Consider that the Mustang
DOES have a automatic reset circuit breaker in the head light switch. It is a nothing fancy thermal CB that trips when it gets hot. And resets when it cools down.
I do not believe it is a CB because if it were, ONLY the head lights would be affected. Secondarly, I don't think it is possible for the CB the heat up, trip, cool down, and then reset so quickly. The period of the event is just too fast.
The single point of electrical failure for the fog lights
and head lights are the head light switch itself and the car's base charging system. There is a fog light relay that could be dropping out if the voltage is dropping to low. But if so, this points straight back to "sagging" voltage and the car's base charging system.
While it is true that HID's do use less current when working, the problem is the
HUGE initial inrush of current needed to start the ballast. The point about a battery being able to supply hundreds of amps is valid. However, what is not known is if corrosion is limiting the available amps. Perhaps this also offers an explanation of how it worked for a while and then started failing.
Another possible cause could be something as simple as corrosion at the small wiring connector at the alternator itself. That is were the power for the alternator's regulator comes from.
In anycase, this issue can be easily answered by a simple voltage test at the battery while the problem is happening. With the shortage of real tests by the OP, the fact of the matter is it could be a great number of things. And all of this is just an "educated guess".
Head light and fog light wiring diagram 85-1 attached for review.