Troublesome Brake Light Issue

Hi gang,

I have a 1971 dodge dart swinger slant six 225 and I have this strange tail light issue I hope you Mopar Gurus can help me with. All my lights work fine EXCEPT for one nagging issue. When the headlights are On and the brakes are pressed the Passenger Side tail light goes out. Turn Signal still works, reverse lights work, and when you release the brake pedal the tail light comes right back on. What could be causing this to happen in just this one condition??? Any help would be appreciated....

Thanks