Couple of questions

[roddster]

Amen on that comment!

[highland green]

the flasher is a Wagner, but not sure of the number. Pete will know it by heart.

[TedS]

Wagner 323 per this thread
https://www.saac.com/forum/index.php?topic=1660.0

[Coralsnake]

Hopefully without LEDs

[JWH]

Here is something that solved this problem for me on my '68. I tried different flashers and still had the problem. I then read that each light bulb socket must have clean contact with the tail light housing for a good ground. I removed the light bulbs and then pulled each socket from the housing. Using a Scotch Brite pad I cleaned the area where the bulb socket contacts the housing for all six. Make sure there is solid, clean contact for each bulb socket.
     As I recall, there also is a ground wire that comes out of the tail light wire loom, and grounds to the tail light panel. Make sure that this connection is clean and tight also.
     After these steps, all three lights lit up equally bright.

[Coralsnake]

If I had a nickel for everyone that said "my grounds are fine" I would be a millionaire

[Bob Gaines]

If I had a nickel for everyone that said "my grounds are fine" I would be a millionaire

+100

[Coralsnake]

As stated the original housings are zinc castings. The bulb sockets have to ground to them. Even then, they are mounted into the fiberglas valence.

In other words, good grounds are not inherent to the original design

[FL SAAC]

1 - You make sure ALL grounds are grounded.
     Clean and assure all paint is removed   
     from all contact areas.

2 - I would still call west coast cougars 
    get their advice as they have years of
    experience

3 - Take it to a good electrician if all
    fails

Best of luck

[68blk500c]

As I recall, there also is a ground wire that comes out of the tail light wire loom, and grounds to the tail light panel. Make sure that this connection is clean and tight also.

That ground wire attaches to the zinc reflector housing (from the factory).  It is a common source of problems.

[427heaven]

So an update on the taillights. The problem persists with both standard bulbs and LED bulbs. Cleaned/burnished ground posts on bulb housings and that didnt seem to work. The inner and middle lights are clear and bright so no apparent voltage drop.....

[TA Coupe]

Did you actually use a voltage meter to see if any power is going to them?

      Roy

[highland green]

It has been a long time since I worked on my Shelby tail lights. I used the rear harness and instructions from Pete. They were excellent. I do remember, changing the headlight switch and the dimmer switch for new ones. cant remember if that had an effect on the tail lights. Im sorry I cant be of more help. My advise is to clean all contacts and put back to stock. Get it working ,then look at improvements ,if still wanted. The correct flashers are a must.

[427heaven]

Update on my blinker problem... After checking out my new system it appeared that I had a standard Mustang flasher and needed the DO-- whatever flasher. I got the ONLY flasher that apparently works with this set up, (WHO KNEW) Not me! Also I found behind the instrument cluster a main ground connection and it was a bit dirty and a bit loose... Cleaned tightened and BAM all was good!!! Thanks to ALL for the help. Much appreciated.

[Coralsnake]

Ah 🤷🏻