2016 grizzly wiring issue

i hooked up my rear seat with led tail/brake light,everything works as it should but today i saw that when the lights are in the off position and any brake is applied the front headlights flicker/pulse,unhook the rear tail light at the connector and the pulsing stops.with headlights on everything works like normal.
lights on with switch and brake light on when brake applied,seems like it is connected properly so i have no idea why the headlghts are doing what they are doing……dealer also has no idea why……..any ideas?
thanks!