my driver side tail light is out

I THINK THAT MEANS THAT THE BRAKE LITES ARE WORKING. THE BULBS ARE TWO FILAMENT , LOW FOR TAIL LITES [ RUNNING LITES ] AND BRAKE / TURN /HAZARD LITES . SOME TIMES THE ELECTRIC GOO , FORGET WHAT ITS CALLED, IT PREVENTS CORROSION, ON THE BULB TERMINALS WILL HELP, IF NOT, IT SOUNDS LIKE A BROKEN CONNECTION SOMEWHERE UP THE LINE