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A few weeks ago, we bought a partially restored 66 Mustange Coup. Car looked great, ran great, interior completely restored, air conditioning added, disc brakes added on front, electronic ignition added, great price....needless to say we thought we'd bought a gem! Its been in the shop on and off since then...carborator issues, replaced altenator, and other issues. Finally got her running great again and started trying to accomplish two little things: 1. install a radio and 2. One headlight was dimmer than the other so we began trying to find the source of that problem.
Long story short, one fixed issue with the wiring has lead to something else quitting. When we actually got into the wiring...what a mess! Partially wired on old wiring, partially on new wiring, fusebox bipassed for some things, naked, hot wires hanging loose...just a mess! We need a lot of help and have already sunk an additional 1500.00 into this car. Anyone have any advice? Now the brake lights won't work properly...will come on but will not brighten when you press the brakes. The signal lights which originally worked, will no longer work. And to make matters worse, when trying to put the instrument/gauge panel back in place, its been impossible to get all the gages rewired properly to get them all working. Any advice would be appreciated.
 

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Grit your teeth - it sounds like your going to get some electrical wiring experience.

They (Painless and others) make replacement wiring harnesses, but they are expensive (500$ about) and the main harness (where most of your troubles are) involves alot of time on your back under the dash. It really sounds like you have a fire hazard on your hands, so it's best to get it done and done right.

PS: Wecome to the club and congrats on your 66 purchase - things will work out, you've come to the right forum.
 

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40 year old wiring is a nightmare. I highly recommend you get a complete new wiring harness. I am using the one from american autowire. it comes complete with new headlight switch and relays for the horn and lights so it should be much more reliable. speaking from experience you don't want to try to sort out that wiring mess. all those old wires are brittle and the insulation is dried out. do yourself a favor upgrade to a modern wiring harness with blade fuses and relays you will be glad you did and you will save a ton of time and money in the long run. after one electrical fire in my car from shadetree wiring repairs I learned my lesson.
 

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This link is in the FAQ section at the top of the classic tech section you posted in:
1966 Mustang Wiring and Vacuum Diagrams

Print them out as big as you can and start tracing/replacing. I am replacing all of the wiring from my firewall forward and have the shop manual and these diagrams printed to write on and make notes as to what each wire controls as I fix it.
I bought colored wire and have been marking them to match the factory colors with sharpie permanent markers (blue wire/ yellow stripe,etc.). Also went up to 14ga. wire to replace all of the factory 16-22ga. stuff.

If your fuse box is currently bypassed I'd look at replacing it with a universal fuse block similar to these with ATO/ATC fuses. It'll make life a lot easier and you can split up more of the ciruits to trace problems:
Fuse Blocks - $50-$100 - Painless Wiring - SummitRacing.com

Wish I would have done it when I had my fuse block out to ditch the glass fuses and get more circuits available. It's up under the dash on the floorboard so you can't see it anyway w/o sticking your head under there.
Jon
 
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