Brake Lights- Weird Issue

[Matador]

I have a 1987 Chevrolet R10 with some brake light issues.

The right rear one works fine. Both tail lights at night work. Both rear turn signals work. The left tail light, though, doesn't work.

I thought this was going to be a broken wire, but the schematic I looked at shows that both the turn and stop functions run off the same wire.

Any thoughts?