trailer marker light doesn't work

Don't really understand your post because the taillight and brake light is one fixture with one bulb that has 2 filaments in the bulb on the rear of the RPod one in each lower corner (right and left) the marker lights (are powered when the trailer is hooked up to the tow vehicle and its driving lights are on) are on both sides of the trailer one in the front (amber) and one in the rear (red) and maybe you have 3 red marker lights on the the above the rear window located on the air spoiler.  Also there should be marker lights on each fender.  If you are talking about one of the marker lights on the side of the trailer brake light power does not go to that light.  The running lights power may have become disconnected or the bulb is burnt out to that side light.  You have to investigate it and not sure about the fixture on your trailer but on mine (2019) the light cover just pops off to expose the bulb and that is where I would start.  Hope this helps