As mentioned the high level stop light is a row of LEDs which apparently are non renewable.
To gain access to it you have to first remove the inner trim on the top inside of the tailgate. Just gently prise it off and you might get away with only breaking a couple of clips. Then you can remove the exterior panel housing the stop light. This is held on with 2 large flat head bolts, 2 ordinary bolts and 2 plastic clips. You will need a T30 torx bit. Although about fixing a non opening rear window I got the info from here:
http://users.skynet.be/sky83161/sven/openingc5.pdf
Once off you can then remove the stop light from the panel.
But as myglaren said problem may be elsewhere.
At least you might be able to test it once removed.