I'm pretty sure it's from exposure to sunlight. When I bought my TT, it had always been garaged and rarely parked outside during the day, so the 3rd brakelight was all black. Only problem was the previous owner had closed the decklid with a golf club sticking out and chipped it. So, I bought a new taillight strip and replaced it (I left the "Nissan" and "Twin Turbo" stickers off because I thought it looked cleaner that way). Also, there's only one part number for it.