Ah, an '84 - from The Era Of Biodegradable Wiring. I'd be checking the loom for chafing and broken insulation, particularly in the cargo area where the loom passes into the back seat.
As Odin said, mismatched globes and high-resistance globe sockets are a problem. As is corrosion of the ground wire and connection point for the tail lights on that side.
Look also at the fuse box. Although if your interior light is working without hassles, so are the brake lights; they get juice from the same circuit.
Do you have / did you once have a tow bar? If so, I'd be checking the connections where the installer tapped into the brake light bus, or what was done with the plug wiring when the tow bar was removed.