As fate would have it, the newest issue of Mustang Monthly arrived today and a reader had the problem of his lights on his 70 mustang going out after extended night time use. They said the circuit breaker in the headlight switch is tripping, causing the headlights to go out. They recommended replacing the stock headlight switch which is a 12 amp unit, with a NAPA Parts brand switch for a 69 mustang which is 22 amps. This switch has a bracket that is used on the 69 and 70 mustang but can be removed if the switch is going to be installed in any other year. No part number given.
I and many others with old Mustangs have had this problem at some point. All the current for the headlights goes through the little ol' headlight switch on your dash; after 40+ years, the switch wears out with use and age, and cannot supply current to today's halogen bulbs without overheating the (now worn out) circuit breaker.
A faulty headlight switch is usually the culprit. I replaced mine and my headlights worked great. I also did a little upgrade to the wiring harness; here is a great instruction on it: http://midlife66.com/wiring/halrelays.html
This upgrade takes all the current load of the headlamps off of your original harness going through the dash and switches, and places it on the new, heavy gauge wiring and 30A relays. Once I did this, my headlights were VERY BRIGHT, like a new car. Wow, what a difference. The only current going through the headlight and dimmer switch after this mod is the switching current used to operate the relays, and is minimal at best.
As others have suggested, get a new switch; if you really wanna make a difference, get a new switch AND do the wiring upgrade. If you're worried about appearances under the hood, I have read others hide the relays behind the headlights or behind the buckets and in front of the splash shields to keep the engine bay looking stock.
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