Former ambulance . Having trouble with rear left lights . Brake lights work, they shut off when either the headlights or turn signal comes on. All three work but not when another function is on . Right side works perfect. Is this a wiring issue or flasher malfunction? I'm confused how they work. Only two wires and the two separate filaments but therethe filament gets lit up every time.

 

There should be 3 wires at the lower bulb socket.
One for tail lamps, one for stop.signal lamp and a ground wire. They did have problems with the ground wire rusting and braking away from the bulb socket.