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You're on it, the original dual filament bulb is markers and turn signals apparently the "plug and play" model gave you only turn signals that were somehow wired to both marker lights and turn signals so when you had your lights off turn signals would work but when you turned your lights on you had constant power to the single filament. What you need to do is sort out which wire is which and go back to the dual filament setup, it sounds like you have both leads there so that won't be an issue.
A single filament bulb will have once contact point in the center and the bayonet connector has equal side pins because orientation doesn't matter, the dual filament will have offset pins on the side to align two contact points properly in the socket, your bulb is currently making contact with both points in the socket so powered by both turn signal and marker lamp power.