tail/ brake/turn socket
If there isn't a good ground then the bulb socket has 12v floating around and lights a totally unexpected filiment, thats how that happens. :-)
Try a wire between the bumper metal and actual socket base metal while the light is on and see if it straightens up.
Either you have a miswired or missing connection or a ground problem to the bulbs, and the above will eliminate the ground possibly being the problem.