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Ya, ebay for the 3rd break light stickers. Cafefull you only get one shot at putting these on.
If you mix a squirt bottle with water, little denatured alcohol and a couple drops of baby oil together and squirt this on the light first, it will allow you to slide it around a bit to get it centered. Then rub it down good and let it dry. Then remove the application paper on top and your left with the vinyl sticker.
This is the formula that vinyl installers use to apply stripes and other graphics.