I had the same problem with the third brake light not working on my 2008
First open the hatch remove the trim around the top of the window. just the left side first then the top middle piece
be carful not to brake the plastic trim tabs like me.
Then you can unplug the brake light and test it with some spliced thin wires to the battery.
If the brake light works you just save your self $150.00
The problem with my light was with the wires that run between the body of the car and the hatch door. Slide down the rubber hose to expose the wires.
The green wire that runs to the brake light was broken probably from opening and closing the hatch. At the same time I noticed both the black & red wires that run to the wiper blade motor were frayed.
I taped and repaired the wires had to add a piece of wire to the green wire because it was to short. The light works great now
If the light did not work with the 12 volt battery test you need a new light about $150.00 new.
To remove the light you well need to remove the 5 nuts that hold the spoiler on. The 5 th nut is under the middle trim, disconnect the window washer hose & unplug the brake light then remove the spoiler from the car.
then there are 4 screws that hold the brake light on to the spoiler.
hope this is of some help.