Something that happens frequently on the old trucks(and cars) with the metal socket and reflector is corrosion will form in the seam where the socket is crimped to the reflector. You can test for this by running a jumper from a good ground to the socket itself, if this brightens it then you can remove the tailight bucket and put a dot of brass on with half the dot on the socket and half on the reflector. Be careful because many of the old sockets are themselves brass so heat carefully. Good luck

Evan