Well problem solved after 3 hours.
Posting a reply to myself just in case someone else has this issue.
I had recently installed a radio under the light monitor above the driver. During installation I removed the light monitor, and had only plugged half the cables back in. This has 4 cable sets - upper left, upper right, lower left, lower right. I had plugged the upper cables in, but not the lower.
The lower cables caused the rear lights to not function.
Here's the weird part - the upper left cable set came loose during the drive, and that caused the left turn signals to fail bus wide AND the horn to stop.
I thought it must have just been a weird quirk. Risking causing more problems, I can replicate the error. Unplug upper left cable set from light monitor, horn doesn't work. Relay clicks, but horn doesn't sound. Cable plugged in, it works.
No clue why the horn is affected by the light monitor, turn signals, and specifically the left ones.
Check your light monitor connections. Because reasons.