brake light inoperable

[golfer Guy]

They left brake light is out on my Boxster. The right one works. I have looked at the bulbs and all of them look good. Replaced the single element bulb with new but that did not do the trick. I believe the three bulbs with double elements in them are the tail lights. Since the right brake light works I don't think it is a brake light component that is out. What do you think the problem could be and where should the search begin?

Thanks

2008 Boxster S