The issue is that the light sockets themselves are not properly or at all grounded. The bulb normally picks up a ground through its case, but in this case it picks up an incorrect feedback ground through the opposite wire in the light socket itself. turn signals stop working when the lights are turned on because a 12-volt signal is now being sent down the opposite wire, which was previously acting as a ground. When the lights are turned off, everything returns to normal. https://autovehicleparts.com/turn-signals-dont-work-when-headlights-are-on/