I tried to search for this issue and came up dry.

It's a 1989 Base Canadian Firebird so it has a row of tail/stop lights, and separate amber turn signal lights. It also has a trailer wiring harness wired in, which may be causing my problems. Only the driver side brake light function has ceased - the running lights all work, the third brake light works, the passenger side light is fully functional, but the driver side brake light does not illuminate. The bulbs are obviously fine because they're dual filament and lighting the tail function...

My plate is very full right now and I drive this car daily... so pretty quickly I'm just going to add my own wire and jump power over from the passenger side for braking. But this car is pretty much 100% original, so before I do that, I thought I would ask if there is anything I should look at that would cause this specifically?

 

Sounds like a bad bulb. One of the filaments is likely out.

 

That's not how dual filament bulbs operate. One filament can break leaving the other still functional, just replace the bulb.

 

There are two brakelight bulbs on each side in the back, yes? If so, and if both bulbs have good filaments, I'd be looking at where the trailer harness is tapped into the car's harness