The electrical circuit is simple. The fuse is #22 and it goes directly to the switch at the break pedal. When the pedal is pushed, closes the switch allowing power to three buls: one, rear combo light left; two, high mounted stop; three, rear combo light right.
If the driver side is not working, I would check the wiring and look for a possible wire cut. Also check for a possible bad ground connection, follow the black wire from the bulb connector.