Tail Light Problem

[Pac06]

Hope someone can help me out. I have a 1972 chevy step side pick up. My problem is that my passenger side tail light, with headlights on, looks bright like the brake is on even though brake is not on. Driver side is working like it should. When I look at the actual bulb and compare it to the working side, it looks like its backwards, meaning the opposite filament of the bulb lights up, not the same as the working one. Hopefully some one can help me out. Thanks!!