Kevs57,
When you look at the lamp, make sure it is the brake filament that is illuminating. This should be the larger of the two. The assembly has to have a good ground to the bed and frame. Make sure the attachment is clean and tight. If the lamp is working, but dim, it might be telling you it can't find a clean path back to ground.
If it is a ground problem with the light assembly, when the taillights are on you might see a dim taillight too.
Does your truck have turn signals? What does the light do on a right turn? If you have any doubt about the lampholder, it might be best to replace it now and see if that cures the problem.
Stuart
