The first place to verify a good ground is always under the tailight housing where the assembly screws into the taillight opening in the bed, clean off the metal surface of the bed and taillight housing where there is contact. Wire brush the screws and use some new star washers. And rusty surfaces do not count as bare metal, and neither does paint. Scratch the paint off with a screw driver where it makes contact. Then add a ground wire from the frame to the box frame. If your bed boards are rotten, and the bed is loose because of it, most likely the bolts are all rusty also giving you a poor ground to the box. It won't matter if you have regular bulbs or LEDs. If you don't get a good ground, both styles will give problems.
