What activates the rear outboard lightbulbs in a 2000 VW Jetta bulb assembly?

I was replacing a blown brake light in my 2000 VW Jetta and noticed during testing that the lower outboard bulbs in the assembly do not activate regardless of what things I tried (backup light, turn signal, head/tail lights on, hazard lights on, etc). I even noticed that on the right side of the car the lower outboard bulb was missing. I'd like to know the purpose of the outboard bulbs so I can either activate the circuit by turning on the correct switch or check the correct fuse. I know that the inboard bulbs (on the bottom) are the backup lights.