Is this just a problem with the rear? Does the left front turn signal work OK?

MS wrote:

Turn signal switch can cause untold problems.
What year is the Mustang?

I had an issue exactly like you describe in my car shortly after I bought it.  It was indeed the t-signal switch.  My advice would be source an NOS one.  They pop up on eBay with some frequency, and usually aren't ridiculously expensive. 

When the led light quit working I replaced the with the new 1157 bulbs, I know they are seating correctly because they won't stay in the socket if not correct.
The turn signal switch on this car cancels about a quarter of the turn of the wheel, I think that the plastic that is made of is really stiff compared to the Ford switches. I was thinking of thinning down the plastic arms to make it more flexible so it won't cancel early. With all these problems it may be wise to just get an NOS switch like TKOPerformance said. thanks

When I turn on the emergency flashers all of the front and rear lights flash, it's just the left turn signal that won't work when using the blinker. I checked the socket again today and it is getting constant power for the park light and off/on power with the turn signal on. I noticed that my meter reads 12 volts but the reading fades down but never completely off, I checked the right turn signal and that is doing the same but the bulb works. I have noticed once I turn the turn signal off the relay blinks a couple of more times before it quits. I never noticed that before. Also the left turn signal dash light isn't working

Is there a picture of the proper location? The internet is full of guesses