Progress made, but still no brake light on left

After looking at the wiring diagram, i can't understand how it could be a grounding issue. The turn signal and brake signal are the same wire to the bulb, so if there were a grounding problem, I shouldn't get a turn signal - but I do.

I have a right hand brake light, so it's not the brake switch.

The wire to the left hand bulb is intact, or I wouldn't have turn signal.

Is there any way to test the turn signal switch? I just can't think of anything other than the switch being the culprit.