You look specifically at where the lamp wire pigtail connects to the trailer frame
OR
If a butt connector is used with an extension, at the butt connector.
Put a new wire end on where the ground wire connects to the frame. Then shine the frame with sandpaper until it is bright. I use KEPS NUTS with a screw. They are harder to come loose. Tighten the snot out of it. Then smear Vaseline over the nut and bolt both sides.
Still dim with a problem?
Replace the butt connector
Still dim with a problem?
The issue is within the lamp's socket