All of my rear lights work except left brake light...

Brake AND signal light use the same circuit - so since the signal light is working that eliminates the wiring, bulb and the bulb socket .. Brake light circuit runs through the signal light switch before going to rear lights .. Strongly suspect failed brake light circuit INSIDE the signal light switch ! Signal light switch replacement is 'more' than likely what's needed.. Brake light wires connect to the signal light switch under dash at lower steering column .. You or whomever is going to be there anyway - when replacing the failed signal light switch ... Good luck!