(Trying this again since I'm an idiot and posted in the wrong section the first time)

Need some help with an odd issue I'm having with my tail lights. I have a '07 4.6L GT, in which I replaced the original tail lights with a set of sequential LED lights. A few months ago while making a turn I noticed the indicator on the dash was blinking fast on right turns. Obviously I checked the bulb on the front and the lights on the rear, but didn't find an issue. The interior indicators were functioning normally again, so I figured it was just a glitch and thought nothing more of it. It happened again a few times, very random, then stopped. Now it is happening for my left hand turn indicators, and is more frequent. The left brake light doesn't come on either. When locking the car, the right tail light flashes, but nothing from the left, and then at random times everything works fine. I've connected the original tail light to rule out the light itself, and still the same issue.

A friend of mine suggested checking the turn signal relay, but I'm not sure where that is. Seems odd that the relay would control the brake lights as well, but I admit vehicle electronics are not my specialty, so I won't rule anything out without checking it first.

I'm looking for any ideas on what might be causing my issue, and any tips on where to locate the turn signal relay.

Thanks!

 

Sounds like the blinker/flasher fluid is due for replacement. I'd recommend synthetic.
Nah seriously, it does sound like there's an issue with your sequential LEDs. Try reinstalling the original tail lights and see if that solves it.

 

As I stated in my original post, I did reconnect the original tail light to test if the issue was with the sequentials and the issue remains. Not a problem with the LEDs.

 

Typically in these cars a rapid flashing interior indicator is the cars way of telling you a bulb is out. It's called Hyperflashing. When you change an incandescent bulb to an LED, the car thinks the bulb is burned out because the decrease in wattage/voltage an LED puts out. Usually you need an inline resistor to "trick" the car into thinking the bulbs are the same wattage/voltage as the original incandescent bulbs.
Since you said the original tail light does the same thing, I'm not sure. Does it happen with both original tail lights reinstalled?? I know when I installed LED switchbacks in my 12's front signals initially without resistors, the front and back both had random flashing and then working. I don't believe mustangs from 05 up have a turn signal relay, I think that is now controlled by the ECU.

 

Oops, missed that.
You might wanna check the smart junction box in the passenger side footwell for any signs of water contamination. The cowl drain plugs can get blocked allowing rainwater to accumulate and seep into the passenger side floorboard via the cabin air filter. It's a common cause of funky electrical issues in these cars.