Okay - I got the problem sorted out ... it was the bulbs.
I remembered that the rear turn signals and brake lights used the same bulbs and I hadn't changed to LED turn signals.
Swapped the turn signals and tail lights and the cruise works fine when I hold the green switch plunger in. The turn signals flash really fast, but still work. I could add a special flasher to deal with that, but I'll probably just go back to standard bulbs.
Really dumb design by Ford in this case.
So at least on my car, you can use LED 3rd brake lights, but not both 3rd brake lights and tail-lights. (Not sure if LED tails would work if I still had 194's in the 3rd brake lights).
I still need to take apart the green switch and would appreciate tips on that, but I figure I can't break it any worse than I already have ...