So found that the down shift button sends 1.9v down the up shift green/blue wire, odd that. Fancy CANBUS is smarter than I am. So the new plan is to use a comparator and a photo coupler to steal the signal to trip the on/off for the cruise control.
The comparator will block the 1.9v and the photo coupler will use the up-shift 5v to momentarily connect the brown and orange wires of the cruise control.
That's the plan. But now I have to leave town for a week so the plan is on hold.
Of potential use to someone out there, I had to learn to clear a fault code. Because of my voltmeter work with the down shift button, the check engine light came on. So here's how you clear it:
1. Jump brown and green wire at the diagnostic plug using a paper clip.
2. Put the kill switch to “run”.
3. Turn on ignition (don't start).
4. Pull paper clip and reinsert within 5 seconds.
5. Watch confirmation blinks at engine light.
6. Turn off ignition.
7. Pull paper clip.