i removed the 3rd brake light (to put on a clear one) on mine and it was not an easy job. what i had to do is pull down the plastic cover from the trunk door. this cover is held in place by clips and in my case some of them broke off. this was the only way to get that W shaped piece that holds the light in place to get out of there since there was no possible way to get it out through the little door that is already in there.
Hope this helps! Good luck