Electrical short in passenger side taillight and center third stop light

The passenger side tail lamp and high mounted center third light is not illuminating for braking or night running light (the former being a two filament bulb, runs from the same bulb housing within the tail fixture, the latter being a single bulb lamp mounted in the lift-gate).

I have troubleshooted the bulbs and replaced and cross tested. Reviewed fuses in the engine compartment ("tail") all of which appear to be fine, and have replaced them anyway.

All other passenger side tail light functions are okay: reverse lights up, turn signals and emergency flashing, all good.

I did a voltage check within the tail light fixture itself at the bulb mount points (checked for corrosion, pried them up a bit, etc), and at the receptacle for the third lamp. No juice is getting to either, but the driver side tail light is functioning perfectly normally.

How to I track my way back up the faulty line to test where the short is?

I just replaced the rheostat (instrument light potentiometer/fader) which works fine now, but I cannot time the failure of this lamp to that action precisely, and do not think that they are related.

Thanks in advance.