If all else fails Read the Instructions

Note: Sequential tail light harness sockets are labeled 1, 2, and 3. #1 is the inner light, #2 is the middle light, and #3 is the outer light.
Step 9. Test turn signals to verify that they function properly.

                   This is what I get for not following instructions.

 I bought a Sequential Turn Signal kit for the new 09 Shelby and installed it. I knew what I was doing so I thought; I put a kit like it in the 05 Mustang with no problems.
 I installed the harness to the right side tail light assembly and tested it and it worked fine. Well all right then, feeling confident in my work I installed the kit in the left side I did not test it, big mistake. I had been driving the car this way for a little while now not knowing of the problem.
 The day of the November toy drive in Vacaville, Pete Mitsopoulos, Lisa, Ileine   Kirk and I loaded all the toys in our cars and delivered them to Mission Solano. After we unloaded the toys Pete came to me and said your left turn signal was out of sequence, I said “oh great” I knew I was had right there and then. I had to see for myself so I turned it on and sure enough the sequence was out.
Some how I had put the number 2 light in the number 3 position and the number 3 light in the number 2 position.
  Thank you Pete for making me aware of the problem I have since corrected it. Also thank you for the nomination I deserved it.