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Discussion Starter #1
This morning my bride followed me and said all was well back there. Later she followed me and said I had no brake lights. Not only are the brake lights inop, but so are the parking/tail lights. Strangely, to me at least, the turn signals still work back there. I would suspect the brake light switch but I don't think that has anything to do with the tail lights. I tried a new bulb, but no joy. Before I tear into it and make things worse, does anyone have an idea of what is going on? As always, I thank you in advance for your help. This is a great forum! CS
 

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Here is a '66 wiring diagram. Take a test light and see if you have voltage coming in and out of the brake light switch. Notice that it continues onto the turn signal switch. That is because the turn signal switch uses the same wire as the brake lights. That way when the brake lights are on the switch can still interrupt the right or left brake light and make them flash. Its possible for a turn signal switch to fail and cause you to have no brake lights.

Notice both the brake lights and tail lights get their power from the headlight switch. That's what your two problems have in common. Both problems could be related or just be a coincidence and you have two separate problems. A cheap 12V test light will allow you to figure this out.

https://www.clanmcmuffin.com/66 Mustang Wiring Diagrams (Colorized).pdf

I don't know what year your Mustang is but here is an example of the headlight switch brands that are available. The Motorcraft could be a better quality switch. We always used Standard brand electrical parts at work and they were good. but that was years ago and the quality might not be as good now. I'm not saying it is the switch.

1966 FORD MUSTANG 4.7L 289cid V8 Headlamp Switch | RockAuto
 

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Discussion Starter #3
Here is a '66 wiring diagram. Take a test light and see if you have voltage coming in and out of the brake light switch. Notice that it continues onto the turn signal switch. That is because the turn signal switch uses the same wire as the brake lights. That way when the brake lights are on the switch can still interrupt the right or left brake light and make them flash. Its possible for a turn signal switch to fail and cause you to have no brake lights.

Notice both the brake lights and tail lights get their power from the headlight switch. That's what your two problems have in common. Both problems could be related or just be a coincidence and you have two separate problems. A cheap 12V test light will allow you to figure this out.

https://www.clanmcmuffin.com/66 Mustang Wiring Diagrams (Colorized).pdf

I don't know what year your Mustang is but here is an example of the headlight switch brands that are available. The Motorcraft could be a better quality switch. We always used Standard brand electrical parts at work and they were good. but that was years ago and the quality might not be as good now. I'm not saying it is the switch.

1966 FORD MUSTANG 4.7L 289cid V8 Headlamp Switch | RockAuto
Thanks. I picked up on your statement "Its possible for a turn signal switch to fail and cause you to have no brake lights." As luck would have it, I recently replaced by steering wheel and perhaps I caused a turn signal problem in that process (although things didn't manifest until today). I don't believe it has anything to do with the light problem, but the horn wouldn't work after I changed steering wheels (and rebuilt the horn ring). I will break out my test light and see if I can isolate the problem. Thanks again. CS
 

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Thanks. I picked up on your statement "Its possible for a turn signal switch to fail and cause you to have no brake lights." As luck would have it, I recently replaced by steering wheel and perhaps I caused a turn signal problem in that process (although things didn't manifest until today). I don't believe it has anything to do with the light problem, but the horn wouldn't work after I changed steering wheels (and rebuilt the horn ring). I will break out my test light and see if I can isolate the problem. Thanks again. CS
 

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Discussion Starter #6
VFG, I thank you for the schematic. Thanks to it, I see that I have no life in my brake switch (according to my test light) and no life in my tail light circuit. The circuit numbers for the brake light is number s 10 and 10A, the circuit numbers for the tail lights and license plate light is number 14. As you say, the commonality is that both of them spend some time in the headlight switch so am I correct to think that my next move is to replace the headlight switch, which, by the way, I replaced just a few months ago. I apologize if this seems like a naive question - I assure you that I am really new at this and appreciate you stepping up to help. Thanks again. CS
 

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My money's on turn signal switch.
 

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Discussion Starter #9
Frankie was right! Because I wanted this to be easy when I should have wanted it to be effective, I now have a new headlight switch and brake light switch (neither of which solved my problem) and turn signal switch which did indeed fix my problem. Now, because I installed a not needed new headlight switch, I face the contortionistic dance to hook the speedometer back up. Why is that thing so easy to disconnect and so hard to hook back up? Thanks to all. CS
 
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