I’m working on a 1968 Camaro base model just replaced all the lights with LEDs. Everything works great when the headlights are off as soon as I turn the headlights on I get no blinkers, tail lights/brake lights still work but blinkers do not.
The problem is likely faulty grounds for the front lights. It could be in the sockets or at the body ground points. Having all led bulbs could be a complicating factor depending on whether they use resistors or not.