Well, there is only one way to trouble shoot at this point, you need a dc volt meter.
Both the left and the right brake lights are fused from the same fuse, if the right is working, the fuse is fine.
The wire for the brake signal on the left (driver's) side is light blue, the right side wire is also light blue.
Check, with the volt meter, to see if there is 12 volts on the left wire when the pedal is pushed.
No voltage, the problem is probably a broken wire,
If you have voltage, the problem is in the left side circuit board (perhaps a contact in the brake light socket), usually a replacement circuit board is the way to fix it (the board for the right side is the same, a swap can tell you if the problem is the board).