I'm with Otto. I believe there are also some cars that use high mount brake lights on both sides without a third in the center.
It's a good idea to have something, given current drivers tendency to tailgate and obscure their view of lower mounted brake lights.
I think there are too many drivers out there who got their license from a box of Cracker Jacks.
But a trip to the wrecking yard could score what you're looking for. A high mount/cargo light assembly from a modern pickup could be mounted above the rear window (opposite where the dome light mounts on an AD - I don't recall if a '59 uses the same dome light location.) My F250 has one that I think could be pressed into service for that application.
