Brake light problem 72 stepper

[Devins-step]

researched here and can't find a thread that gives an answer to my problem. As briefly as possible: Tail lights work, signal lights and emergency flashers work, brake lights work. When the brake is on and the left signal is on right brake light is good but when the right signal is on with the brake there is no brake light on the left side. So when I turn on the right signal and hit the brakes it seems to have no brake lights at all.... Thanks in advance for your help guys.
Dan