Are your trailer lights actually on?
I have one of those cars that allows me to set the lights as ‘automatically on’ whenever the engine is on – which gets me lots of flashes but that’s another story. I was testing my trailerboard the other day and made the shocking discovery that the main taillights don’t work with the ‘auto-on’ setting– even though all the other lights work as normal and the taillights work ok when the car lights are on the normal setting. Might be worth checking how yours works…
Fleet Captain, Datchet