1973 Chevy Truck Blinkers

Here's a better description. I have a 73 Chevy stepside. Turn signals work fine. All the lights work fine. But when I turn the signals on and then turn the lights on at the same time, the turn signals stop. When I turn the lights back off, the signal starts again. Same thing if I turn lights on then signals, I get no signals. Same on both the right and left. I narrowed it down to the tail light bulb. When I have the lights on and turn the signals on, the signals don't work. When I pull the tail light bulb out, the signal works. The left tail light is grounded to the metal plate in the plastic housing. The right side is grounded to the light bulb holder inside the plastic housing. All the relays and fuses are good. Can anyone help?