yeah its the lights
there is this gap when you open the boot lid on the seams
jus get silicon and seal it
i had the same problem and that solved it
but because you seal that gap, when it rains water will puddle up in that area and alot of dirt and shit gets trapped there and the silicon goes all black. Good idea to give a good clean and wipe everytime you wash the car.
I also think this problem might be related to aftermarket spoilers as there are holes in the boot or there is a slight gap for water and crap to go under the wing.