I have a 2015 mustang gt that has recently started having issues with the tail light sequential on the passenger side not lighting up on the bright setting (for brake and turn signal) but will operate the dim setting when the lights are on. It is not occurring all the time, I can have the car sitting in the garage and have it not working, go down the street and have it working again, then go to another store and have it out again.
Any ideas what could be causing this? Recommendations or links associated. Pictures attached.

 

Bulb on its way out or a bad bulb socket. When it doesn't blink, all the way over does the turn signal blink faster on the dash? If does, then it's one of the thing I mentioned, if not, no idea.

 

Yes, it blinks as if I light is out, however these are led boards instead of normal bulbs and all of that is internal (unserviceable). I'm wondering if there is a wiring issue somewhere or something.

 

So you have a harness addition for the sequentials, or is it built into the assembly?

If it is built into the assembly, well, you're in need of a new assembly...

If it is a harness, remove the harness, back to factory, see if the light will
bright when it is supposed to, if not, you lost that portion of the circuit
for the LED panel in that lens. The difference between LO and HI on LEDs
is voltage application to the same LED panel. The parking lights, a resister
between the car parking +12 power lead and panel is used to LO light the panel.
Brake or Signal, the full 12V is applied from the car side wire to brighten the
panel.

So, either the car side Brake/Signal wire has a bad connection, or it terminates
at the LED panel, and does not get through because a diode or current limiting
resister on LED panel board has failed.

Something like this:


So, as you see, the Parking light power is reduced by the Resister so that panel is LO.
When Brake/Parking is applied, no resister for full brightness.

The diodes keep current from going back through, so when the brake/signal is
on, power is not fed back through the parking. With Parking on, no flow
back through the Brake/Signal.

IF the HI diode failed, you get no HI lighting. Some use the litle
fuses you can't replace after the diode... IF that goes, no HI either.

CRUDE diagram, I was not going to design the whole panel....

 

Uhh i remember issues with pinched wires in the trunk, but don't remember the details.
Search on here or online.

 

It is the stock harness, though referring to the comment about pinched wires, I recall reading that as being an issue with the backup camera (which is also intermittently going black or sliding the image up and down periodically) but I could not find where these pinched wires they were referring to were ere located. Think this may go hand in hand?

 

Had a similar issue with my 2015. After doing some research on a lot of Mustang forums I found that this can be a battery issue. My car was not driven much in the winter which resulted in the battery getting just low enough to cause this problem. Ford has noted this as well and I even read where there were possible upgrades in software to address this. Keeping my battery fully charged eliminated the issue for me. Hope this helps.

 

So i just had this exact same issue and fixed it myself after a few hours of self diagnosis. As it turns out the Ford Mustands from 2015-2020 all have a fail safe in there computer system that is similar to your cellphone where once the battery starts to show signs of loosing its charge it will begin to shut down features the car has that are not essential to the cars basic function. so i tried to swap the battery with a new one and it reset everything in the computer due to letting it sit for 30 minuites with no battery in it then because the computer recognized having proper steady power from the replacement battery it restored all functions to the car includidnt the the rear signal lights. saved me over 1000 as the dealership immediately wanted diagnosis of $175 and said it was most likely the taillight would need replacing and they recomend doing both whish was over $700 to replace and that was there starting point. So I would highly recomend trying an alternate battery to atart as for me where i live a brand new battery was $123.78