So I have a trailer with an aftermarket tow bar.

When the trailer is connected sometimes (mostly) the right indicator will not work, and inside the car it's going super fast.

Bulb is fine - sometimes works.

Wiring is ok I think, checked both plug and socket, and the wiring seems fine.

AFAIK the car is fine - I have previously used it with another trailer and it worked fine.

What should I look at?