You will need to trace the wires coming out of the lamp and check the connections and grounds.
This pdf should show the wiring somewhat, it is for a 72 but should be close.
http://www.adrive.com/public/a687ed651f1e3dab2be18d4be1a1c54a32a5042c3198ac29d9a5292b777f9e77.htmlYou likely have a loose connection, corroded ground, or a broken wire.
The ride IS the adventure. The destination is just to get gas!