have you replaced the bulbs or screwed with anything back there??? I'd check the contacts in the bulbs (sometimes the contacts on the bulbs can get bent) or I'd check for frayed wires that are grounding out.......or just bring it to the dealer...that is what the warranty is for isn't it
Check for a wire going into the harness. Make sure there are no wires hanging. Follow into the cabin. If you have people sit in the backseat a lot maybe there is a problem. Also check and make sure the vent tube is not pinched. If there is water in there and it can't get out it will cause a problem.