I have an issue whereby both reverse lights are working but only with a single bulb in at a time, if both have bulbs then only one light works. My research has pointed me to the ground wire being the culprit and to inspect the bolt in the trunk where the ground connects. I'm seeing conflicting information as to where this bolt is though. I think it's behind the trim in between both light assemblies but if someone could confirm, I'd appreciate not potentially having to pull extra pieces of trim off looking for it.

 

The ground is located center of the trunk, underside of the shelf.

 

I wish I could zoom out on this because that image is throwing me somewhat. When you say the 'shelf' is that the inside of where the "Civic" reflector/garnish plate is on the outside?