Odd brake light operation

My driver side brake light seems to work perfectly that is till I turn on the headlights. With the headlight on the tail lamp is on as would be expected however when I press the brake pedal both the (tail) lamp and brake lamps go out. Release the break pedal and the tail lamp is back on! Bad bulb or bad wires?

Thanks in advance for any replies!:sos:

Bad ground, the taillight is grounding through the brake/turn signal (second filament) and when you turn those on you loose ground, 12V on both terminals -> lights off.

Seems to simple! I'll make up an new ground wire and give it a try. Thank you!!