1. Checked the fuses.
2. Reseated the connectors.
3. Examined the string for breaks / shorts.
4. Removed each socket from under the PF, and removed the bulb - looking for a short across a socket.
5. Traced back through other connectors.
6. Said hmm and checked voltages at J121, which I should have done much earlier.
7. Replaced the connector at J121. No change.
8. Asked the ceiling WTF was wrong with my GI.
9. Lamented that I would have to change big block connectors under the PF.
9. Realized I never actually did #1 at all, even though I meant to start there.
10. Did #1, and found a blown fuse.
11. Replaced the fuse.
12. GI is now perfect.
Sometimes I can be absent minded.