Its pretty easy to replace the third brake light. Its held on by four nuts. First you need a pry tool to pull up the (8 ?) expanding nuts that hold on the trunk liner carpet on the trunk lid. IF your car has a EU warning triangle you will need a T25(?) torx to remove the two mounting tabs. Then you will see the four nuts that hold on the brake light lens.
Here is what the 3rd brake light lens looks like...
http://bmwetk.info/parts-catalog/prd...622/63/63_1055