Funny you should ask, but I just happen to have a circuit that should do that (LTspice sim below).I have a vehicle that shares the same Brake signal as the turn signal. I just need to filter out the Brake and only output the flashing turn signal.

Don't offhand see how to do that.So is there one I can just do for the single left or right side? seperate harnesses?
Ok I get you now, thanks for the clarification. I think you will need to separate these three signals (left blinker, right blinker, and brake) in the existing wiring harness. "Filtering out" as you propose is not simple or fool proof. For something safety related like brake lights you really want fool proof. You need to find where the brake switch is on your car and where its signal goes. Find where that signal merges with the lights circuit and disconnect it, run it just to your new brake lights. Then your turn signals should only come on when the signal is selected, not also when brakes are pressed.My apologies, so to explain it better. The tail lights on the car have is only one light. When you brake the light is solid. when you turn the signal on, it will flash. I'm installing a set of aftermarket tail lights. on the taillights themselves the brake line and turn signal from the vehicle harness will only turn on the turn signals. So when I brake, the turn signal will light up once and turn off. when the turn signal is on, then it will flash. i can setup the brake line to turn on the brake light. but how do I filter out the brake line signal so it doesnt interfere with the turn signals. Hope that makes sence.