[mr0077]

I'm not sure if there's a more elegant answer, but the "bright" lead feeds the brake filament, and the "dim" lead feeds the tail light filament in the bulb. Put a bulb in the socket (it will only go in one way), ground the case and put 12V B+ to each pigtail and mark the one that is the brighter of the two.
There's probably a better way to tell on sight, but the above will work.
Good luck