No fault codes outside it being unhappy with the S2 cats.

When the lights are on and the right blinker is active the right rear tail brake filament turns on and off about every 7 blinks at a reduced intensity. After the the blinker is cancelled a bulb out message is thrown.

The bulb holder is new, bulbs are new, CECM has been replaced with "working condition" used module which acts similarly as the old unit. Cannot find an issue in the wiring by basic diagnostics.

Car does have a Curt trailer light controller that has not caused issues in the past.