What usually happens when they get real bad is where the bulb holder socket contacts the metal backing it burns a hole through the metal and melts the plastic behind it allowing the metal to pull in.
What you could do is either use this as an excuse for some different tails, or repair them by pulling the plastic back out flush if need be and filling behind it with epoxy to keep it flush, them sanding out the hole in the metal and filling it with solder, then file flat.
I am actually trying to get a pair of the 04 and up tails that are burnt through the shop as they are replaced to repair and smoke/tint for myself

My trucks but end could use an update too! (If anyone has those burnt housings and hasn't thrown them away yet, I'll gladly take them off your hands)