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Day driving is great. Night driving has become aggrevating. If I turn on the radio, then the headlights, the radio shuts off and then comes back on. With the headlights on, and my foot on the break, the radio sometimes does the same thing, and my tach will go crazy. I realize the headlights are the common denominator but any ideas what the issue could be?
 

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My first advice is always to check grounds since this fixes 80% of problems. Next, the headlight switch is a notorious weakness in the Mustang's electrical system. If you have never replaced that switch, now is the time.

I suspect the ground is the problem, since whenever you turn on the headlights, you are asking for more amps than that ground can handle, so it cuts out. You have two grounds on the radiator support for the headlights, and a ground behind the instrument cluster for the underdash harness.
 

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Day driving is great. Night driving has become aggrevating. If I turn on the radio, then the headlights, the radio shuts off and then comes back on. With the headlights on, and my foot on the break, the radio sometimes does the same thing, and my tach will go crazy. I realize the headlights are the common denominator but any ideas what the issue could be?
Back in the 80's I had a 1971 Torino with problems like your having,
everybody that looked at the car couldn't find the problem.

Then one day I was talking to an old mechanic about it.
He opened the hood and said there's your problem, (you need a negative ground on your battery)
He put one on for me and problem solved.

Hope this helps
 
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