[Porboynz]

All to do with the earth return for that rear light cluster. If the earth return path is missing or high resistance then when the head lights are on the rear light finds an earth return path via the brake light filament, through the wiring and via the earth in the working earthed rear light cluster on the passenger side. When you try to activate the brake lights there is no return earth path. Confused? Its simple to run a temporary connection from the faulty rear light cluster housing to any convenient earth point, maybe use a battery jumper lead or something similar. Then you need to make a permanent earth repair.