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1969 Mustang 428 Cobra Jet
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Discussion Starter #1
I've committed myself to restoring my '69 Cobra Jet, and I'm starting with the wiring. The wiring was messed with before I got the car, so it doesn't appear to be completely factory. Each tail light has a ground wire attached that doesn't appear in any diagrams I found, but I tested with a multimeter and it appears each one is grounded (If this info helps). I realize this has been posted alot, but I'm not sure where to start. All other lights function normally (minus a headlight and reverse light bulb that are out) except for the left rear brake light. It lights up when the headlights are on, but doesn't light up for braking, the turn signal, or hazards. The front left turn signal also works normally, and there's no problems with lights on the dash. The left bulb worked when I put in in the right side but not vice versa, so the bulb's not the problem. I'm unsure whether (or where or how) to try to find a ground elsewhere, replace the signal relay, or check the turn signal switch.
 

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1969 Mustang 428 Cobra Jet
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Discussion Starter #2
I should also add that my father was trying to get it to work in the past by moving it around in the socket, and it would work for a short time and shut off again, but I couldn't get it to work. Yet it's already grounded...
 

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Remove the bulb and use a test light or a volt meter to probe the two buttons in the bottom of the socket.
One should light continuously when the taillights are on. The other should blink when the turn signal is on.(or on while the brake pedal is pushed).

If the button has power to it and the bulb is good then the problem is the fit of the bulb in the socket.
If there is no power to the button then the problem is in the wiring upstream, most likely the turn signal switch.
Based on your second comments my money is on the socket being jacked.
 

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1969 Mustang 428 Cobra Jet
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Discussion Starter #4
Remove the bulb and use a test light or a volt meter to probe the two buttons in the bottom of the socket.
One should light continuously when the taillights are on. The other should blink when the turn signal is on.(or on while the brake pedal is pushed).

If the button has power to it and the bulb is good then the problem is the fit of the bulb in the socket.
If there is no power to the button then the problem is in the wiring upstream, most likely the turn signal switch.
Based on your second comments my money is on the socket being jacked.
I couldn't get a voltage reading on the left signal (while I did on the right), and considering it has connection and voltage for when the headlights are on, the socket appears to be ok. I'll investigate the wiring connections from the signal switch now. Also (before I tested), the signal relay stopped working after I took it off and reconnected, so I replaced it. The left rear still didn't work, ruling that out. If it comes down to having to replace the entire switch, where's a good place to get a replacement? It's looking expensive already.
 

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Definitely check sockets for voltage as described earlier.

Keep in mind about 2 wire sockets, most are set up for 2 elements... park/turn.
Neither should be grounded.

There are exceptions like a door light.
Find a wiring diagram for your make/model and options.

I’m not familiar with ‘69 wiring, I am familiar with inexperienced people grounding wires thinking it might work.

It may make that one light work, but cause issues elsewhere.

Starting in ‘68, the side marker lights will backfeed and make trouble shooting even more FUN.

As I recall, neither wire for side marker lights go to ground at the light.
If either is grounded, it will cause issues.

Although it might be a pain..if wiring is unknown, if you end up chasing your tail, take out all bulbs and verify circuits.

Hope this helps.
 

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1969 Mustang 428 Cobra Jet
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Discussion Starter #6
Definitely check sockets for voltage as described earlier.

Keep in mind about 2 wire sockets, most are set up for 2 elements... park/turn.
Neither should be grounded.

There are exceptions like a door light.
Find a wiring diagram for your make/model and options.

I’m not familiar with ‘69 wiring, I am familiar with inexperienced people grounding wires thinking it might work.

It may make that one light work, but cause issues elsewhere.

Starting in ‘68, the side marker lights will backfeed and make trouble shooting even more FUN.

As I recall, neither wire for side marker lights go to ground at the light.
If either is grounded, it will cause issues.

Although it might be a pain..if wiring is unknown, if you end up chasing your tail, take out all bulbs and verify circuits.

Hope this helps.
I checked each button on the left side, and neither received voltage except for when the headlights were on, meanwhile the rightside acted as it should. For some reason they're really obscure online, but I managed to find a 1969 diagram and it shows ground wires on each side.
 

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I checked each button on the left side, and neither received voltage except for when the headlights were on, meanwhile the rightside acted as it should. For some reason they're really obscure online, but I managed to find a 1969 diagram and it shows ground wires on each side.
The socket is the ground. There are the two leads on the back of the socket: one for the tail light filament, the other for the brake/TS filament. The bulbs (1157) have the two knobs on the bottom that correspond to the two leads. The two offset posts on the sides of the bulb casing make contact with the socket (ground). They're offset to ensure that the knobs on the bottom of the bulb make contact with the correct leads.
 
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