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I think they started putting 3rd brake lights on cars around 1986. I don't know about trucks. You could probably get one off of a newer truck and make it work. It would help to already have a cargo light. You would then just have to wire in a signal from the brake circuit.
btacarlton, you are correct, and I remember seeing the cargo light sold in one of the FTE sponsors for 73-79 Ford trucks (national parts depot?), it would also be cool to have the third brake light and cargo light.
I would thing you take a new truck cargo/3rd brake light and maki work. I had a 94 Ranger that I liked the style of the cargo/3rd brake light from. You would just have to give it power from the brake switch and the dome light.