Hey Fred, I don't know if I'll ultimately be of much help, especially because I always seem to have a case of CRS whenever situations like this pop up, but I'll do my best to recollect what I went through to find my problem, even though it was the blinking light syndrome.
I am assuming that the light that you are speaking of that is on steady is the orange CC light, and when you say it won't set, you mean that the green SET light does not come on at all, as well as the cruise not setting?
So, with a normally operating CC, there are 5 different ways to disable it. The clutch, the throttle, front brake, rear brake, and the one most don't realize, changing gear without the clutch. I mention this last one because I "think" the other 4 ways all have a switch of some sort. I say "think" because I'm not sure about the throttle roll-off. I just know that when it gets twisted to the off position when on cruise, the cruise is disabled, although I don't know why exactly.
And I REALLY have no idea how the 5th way that I speak of works. All I know is that if you have the CC on, in say 5th gear on a slight up hill, and you have a little tension on the shifter as though you want to go into 6th, but without using the clutch, as soon as you crest that hill and the transmission relaxes enough for it to slip into 6th gear, the cruise disables. Why, I don't know, but it does, so something is signaling the ECU.
Since you have changed out both the ECU and the left hand switch gear, I would rule out the clutch switch, and the no clutch gear change as being the culprits. For the time being, I would also rule out the throttle roll-off, because, like I said, I'm not sure if there is a switch controlling that, or if that is a signal from the FBW rheostat that goes directly to the ECU.
That leaves the two most popular culprits, the brake light switches. MY experience has been that when the cruise light goes blinky, it is a malfunctioning of the microswitches in the FRONT brake light switch. The reason that happens, I "think", is because the microswitch that operates the brake light gets stuck on, and I "think", since it is not supposed to operate until moments after the microswitch that disables the cruise is triggered, that it sends a fault code, as well as making that light blink.
I don't think the rear works that way. The reason I say that is because when I tried to listen for the same two audible clicks that the front brake switch provides, there are none, at least not from the switch by the pedal. However, if you turn the key on and then depress the rear brake, a single click can be heard coming from something behind the right front upper shroud. To be fair, that's as far as I ever got, because my problem was the front switch, so I never dug any further to find exactly where that click was coming from. If I had to make a wild ass guess as to what his problem was, I would be concentrating on whatever it is that makes that clicking noise, because the only time we ever hear of a CC light staying on solid, and the CC not setting, is from the rear brake light being out of adjustment, which of course is the first thing you checked, right? LOL