If it worked before with no problem, and all globes are fine.
You will find you have a bad earth, for example if you turn your indicator on the brake one brake light will flash and the indicator glow.
So clean up the tow ball coupling, try a jumper wire from car to trailer (extra earth lead). Trailer needs to be on the car for a good earth.
If your chasing wires un screw both car plug and trailer plug and compare the colours, if there all the same, there should be no problem unless some one did a bodgey colour code.
If all fails try a test light at all the globes test seperate indicators, tail. and stop lights.