Do you mean its faded? I havn't had the pleasure of seeing the interior of a new mustang, but if its plastic coverd you can get a can of spray interior dye the same color and spray it on and it will be good as new. If its not faded to get a replacement look in a salvage yard or your local ford dealer. Or you could just buy a trick billett LED 3rd brake light and replace it.
It is the stock 3rd brake light on a 99 mustang that is built into the trunk lid. I should be able to replace it from Ford, but I would like to upgrade to one which the top was not plastic.