Weird tail light issues

[Cobra Jet]

Shit, I had a complete blank when responding and forgot about this...

Check out this thread regarding brake light issues... it might be applicable to your S550, the issue seems to be present more so in the 2015-17’s:
https://www.mustang6g.com/forums/threads/brake-light-switch-near-pedal-failure.122100/

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[Rapid Red]

Yes. Read only. Headlamps and front turn signals function as they should.

How about the hazards up front?
How about tail lights, aka parking lights are they working out back ?

It the hazards up front and tail lights are working, .

This suggests the problem could be a plug on the steering column, or with the turn signal switch. If no one has hacked the electrics for some after market stereo. I'd suggest to take to a dealer or someone you trust to look at it.

 

[Rapid Red]

Most likely bad BCM module.

Post 25 of this thread may be helpful for diagnosis as well as other responses in that thread:
https://www.mustang6g.com/forums/threads/tail-light-out-then-back-on.101551/page-2

OP consider the above post good read ... could save you a lot of troubled. Mostly cause it seems to be a factory part problem.

If it is a BCM module and there should be a way to test. Replacement of it will cost MUCH less than to have someone troubleshoot it.

You have some good facts and can ask the right questions.

 

[Crowdkiller_S550]

I had issues with mine for like a year, but turning the car on and off would usually fix it. Until one day it wouldn't anymore. Took it to the dealership and they said that my bcm had a code on it. They just reset it and now my lights work fine. Costed me $120

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