Tail lamp out - but which bulb?

I have an odd issue with my 2016 left tail lamp. When the headlights are engaged, my left tail light only lights the upper "half" of the tail light lens. The lower "half" is dark. Compared with the right, both halves are lit up fine.

The left brake and turn signal lights, and backup lights are all fine.

To illustrate the problem, see the picture. The part I have highlighted in yellow is the only part lit up when my headlights are on. The part highlighted in green is dark. On my right side, both halves are lit.

The parts diagram shows only 3 bulbs #5, #7 and #9. I would think that if anything, bulb #7 would be the culprit, but it seems like the entire envelope of this bulb is clear, and it's not like half the glass is shaded. What am I missing?

Mine is a 2016 SR L4 base. Stock. No mods to the tail lights. The only thing I have added is a

$89 Tekonsha 118685 Tekonsha T-One® T-Connector Harness, 4-Way Flat, Compatible with Select Toyota Tacoma

Tekonsha 118685 T-One, which works fine. Didn't have this problem til a few weeks ago.

 

Take the bulbs out and inspect them, whichever one is burnt, replace it. EZ

 

It ended up being one of the two filaments of the middle bulb. Thanks, the arrangement confused me for some reason.