SOLVED:
It was the thermostat.
Now.. one of the first things I did was bypass the Tstat by shorting those two wires together. So..??
But here's what pointed me back at the thermostat.
I drove the truck around next to the furnace. Got out the jumper cables.
I injected my own fake 12v call signal to the control board.
Saw a spark and the fan popped on.
I removed the fake 12v. Furnace left blower on for about 1 minute and back off.
So I pulled the Tstat (2-wire plus some anticipator thing that I didn't think I used.) and started trying to figure out this chinese puzzle of little contacts and plates.
Turns out the antipator is in the circuit.
The anticipator looks like a little round circle of (thermal?) wire with a little arm that you can put at any position around the circle to create different ... anticipation times.
This little arm needs to be ON the wire circle. NOT in the parked neutral position.
Somehow the little arm ... maybe vibration?...had gotten to the parked position.
The continuity back to the furnace seems to go through the anticipator.
