02 Suburban trailer wiring issue

[WrenchMonkey]

For those who don't know, I've got a 2002 Burb 2500 4x4 8.1L. I like it.

I've got no trailer brake lights. I do have both left and right turn signals. So everything on the trailer from the plug to the bulbs is good, the issue is in the truck.

The wiring diagrams I've found say the trailer brake light signal comes off the third brake light. That is working.

So the signal is getting lost somewhere between the CHMSL fuse and the 7-way plug under the bumper.

I can't find a wiring diagram to show how the three signal (Left, right, brake) gets converted to two (just left and right). There's a "Trailer Brake Wiring" box (assume a relay) on this diagram of the fuse block under the dash, but what's actually there doesn't match the diagram at ALL (see pics).

Additional clue, or distraction: The truck's hazards don't work either. Replaced the flasher, no help. Assume it's the switch and unrelated, but thought I'd mention it.

Any suggestions? Thanks.