The side markers can’t blink both headlights on and off with LEDs because the polarity of the socket is changed when it’s one cs the other. My car has turn lamps in front, and they illuminate with park lamps, but switch back with the 3157 mounted forward in the same housing. With an LED, I CAN REVERSE The bulb and change WHICH function it lights for, but not both without more wiring. A simple solution is to wire the bulb to receive power when the front does, but that means not having a legal side markers on a square. My car would still actually have a side marker since the lamp wraps around. You can see about getting a dual “filament”bulb that you can mount in the side marker and then running power for turn to one wire and power to park lamps to the other, then it will increase brightness and blink with,not opposite, your front lamp.