Sure you have sorted it this by now!! but for anybody else having the same problem the third brake light is supposed to just pop out however it is a very tight fit and the inner edge of the hole it sits in catches / digs in to the plastic light fitting!! the more you push the more it digs in!! so with a bit of persuasion and a very thin flat blade screw driver I got mine out but did have to bend back a little the inner edge of the hole the light came out of, you can do it!!