I do have ideas, which is that you have a ground problem or a short in your wiring. If the marker lights are okay on one side but not the other, your signals are getting to the trailer from the tow vehicle, so we can rule the tow vehicle's wiring out. A short or damaged jumper wire would be the most likely problem.
I have added links to help articles on both trailer wiring and troubleshooting trailer wiring I recommend checking out. view full answer...