The likely causes are:
- there is a problem with the interconnecting plug for the passenger side brake lights
- there is a problem with the grounding connection of the passenger side brake lights
- the brake failure sensor has failed
Easy things first - check the right side tail lights. They share the same ground as the brake lights. If the right side tail lights work its not the ground connection. The left side brake lights get their power through the right side sensor module (see the diagram below). If the left side is working correctly that pretty much gaurantees that the interconnecting plug is OK; but, check the plug anyway. Make sure that the plug to the sensor module is OK. As a final test, you can by-pass the sensing element in the sensor by adding an electrical jumper from pin 4 to pins 5 and 3 on the sensing module. If the brake lights start working that pretty much confirms the sensor has 'snuffed it'.
Something is a bit off with your warning light because when the brakes are applied the warning light circuit requires a ground through the sensors to stop it from lighting up (it is supposed to be fail safe that way). Does your warning light power up when you first key on? If not, you could have two problems, right rear brake light problem and the dreaded instrument cluster capacitor failure. Don't mess with cluster problems because they have been linked to vehicle fires.
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