I know this has been discussed a lot. I got some info from some "Closed" threads so I decided to see why my daughters Third light was out.
Hopefully I get these pics uploaded.
1. Take two screws off third brake light
2. Pull cap off of the Wire side of the brake light bar. The small circuit board with LEDS will slide out.
3. Look in the lower left side to see if a small component is missing and just two small wires are still there. see pic
4. look at back of circuit board to see the Triangle with line that indicates the direction of the part you need to replace. It's called a Diode.
5. I had one that was overkill but it was handy and I soldered it in. Most diodes do not have a triangle they just have a line on one side of the component. make sure that Line is closes to the wires that are coming into the Circuit board.
Not sure why this Diode popped but it's fixed now.