Most likely you'll need either resistors or capacitors. Going back more than a few years here: Something like 1500 - 2500uF caps worked if I remember correctly. (They also work for the flickering caused by adaptive brake systems.) Failing that load resistors or a bulb in circuit. The latter are a pain because of heat problems (or bulb breakage.) so placement is important. i did a quick test with resistors once and was surprised how hot they got.
My LED rear lights have loading system built right in which I believe is a cap for each side.

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