Hi,
I think that this wiring diagram is correct. Apologies if it is not.
It shows that to operate the driver's side brake lights the power comes into the light on the yellow wire from the left turn signal indicator switch contact when it is not operated. It comes into the right side light on the dark green wire from the right turn signal indicator switch contact when it is not operated. These switches are located in the turn/hazard/headlight switch assembly. Presumably this is connected to the stalk control on the steering column which controls the turn/headlight/hazard light functions.
Brake light power comes into the unit on the light blue wire, through the switch at normal, to the centre point between the left and right turn switches, splits to go through the switches at normal to go to the lights on the yellow (left turn/brake) and dark green (right turn/brake) wires. See image.
I think that the left turn switch contact at normal, (operated is OK as you say the turn indicator light works) or the connection between the left and right contacts (as the right brake light works OK) is faulty.
Apologies if you know how to read a wiring diagram
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
2