Each taillight should go to a common ground. If you are use one, then all the better.
If you have to, use an OHM meter to find the right wire.
You might want to consider soldier each connection then shrink-wrap. If you can use the bonding type wrap even better.
Tests should be done with a 12V Power Supply at each light. If they are dimmed different and tied to a common ground, then try bulbs.
LED lights may be a challenge in that they may load differently. If there is a challenge with this, try a loading resistor or "other" load that can be screwed to the frame for cooling purposes.
Hope this helps.
( spelling optional )