The solid YELLOW wire travels from the main trailer plug to the left tail light. Check the yellow wire for any pinched or sliced wire. Also check that the white ground wire behind the tail light is making good contact with the metal tail light bracket. If equipped check both bulbs (CEC 1157 & C193) in tail light assembly. If there is excessive corrosion in the light fixture it is best to simply replace the the tail light. LED Tail lights are not serviceable and simply need to be replaced if some of the diodes stop working.