Driver side brake light stopped working

Check for voltage to that brake light while a helper presses the brake pedal. With no wiring diagram to look at, just use a test light to probe each terminal of that bulb's socket. If no voltage is found there, suspect the turn signal switch. We know the brake light switch is working because the right brake light works. If the left rear turn signal bulb is the same one as for the brake light, and the left turn signal works, that proves the bulb and wiring are okay.