Jeff,
A "splice party"! I wouldn't hesitate for a moment to run a couple of new lines. The color isn't important unless you are a purist.
Go back to the nearest switch or terminal and start fresh.
You can attach each wire after you run them to the new taillight. The brightest connection will of course be the the BRAKE circuit.
Judging from your ability to troubleshoot this problem, you DO have some type of meter or tester to verify the presence of voltage...this item would be invaluable as you can't readily see the little electrons dancing around (at the party).
Stuart
