Drivers side one comes on when lights are turned on, goes out when brakes are pressed, and also goes out when left signal is turned on.
Make sure the bulb is a dual filament (1157?)
Make sure it fits in socket properly, so bulb contacts are touching proper contacts in socket.
Make sure the bulb is ok, sometimes the two filaments can touch and screw up operation.
Try with a known good bulb.
Good luck.
Pete
