Might be easier just to run a new wire for that particular filament from the tongue on back. It wouldn't necessarily need to follow the same path as the factory wiring. It would save a lot of head scratching at someone's hourly rate.
Someone recently posted about their interior wiring developing a break in the insulation. If that happened somehow with your tail light wiring, the problem could be that the current is being grounded. Have you looked where the tail end wiring comes out through the rear frame crossmember and goes up into the space behind the rear compartment? On my 2011, that happens below the left taillight. There's no grommet there, I think I recall, and it's a good place to develop a short.
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator