Friend's classic WRX - no wiring mods - as stock.

Lights off - press brake pedal and right rear brake light illuminates and left rear light (not brake light, just normal rear light) illuminates. Left rear brake light does not illuminate.

Lights on (sidelights, dipped beam or main beam) - press brake pedal and right rear brake light comes on as well as right rear light already being on, but left rear light goes out and left brake light does not illuminate either.

Any ideas as to where to start looking?

 

Rusty connector in the bezel somewhere, common problem on the old mk3 and mk4 escorts, I was forever fixing them on my first car, prob due to a faulty seal around the light unit or a leak from tailgate.

 

As above but also check the earth points are good.