One of my sidelight units at front of van went faulty during ITV( MOT ), indicator worked, sidelight worked intermittently ( ok b4 test, not working when arrived), found that the earth bar between indicator and sidelight at rear of unit had corroded inside rivet points, giving intermittent light faults due to poor earth contact within unit itself.
Wire brushed contact joins at rivets and put a non- dry joint solder blob between the two. Havent had any more intermittent light fails since, didnt need to get a new light.