Well, the good news is it is not DSP related. I still have 2 working DSP amps and 2 spares......
After having zero issues with the head units, I now have one on each car!
Now that it is cold and wet here (Getting down to freezing at night) I have found that the Borrego's stereo now comes up with "Telephone"
and refuses to do anything.
On Bolt, the stereo had a few hours where you could not turn it off, change stations, or volume. You could shift it to CD to shut it up.
Next day, dead as a doornail......... No display, no response to anything. 3 days later, it is back and after about 15 min of being able to control it, it is now on, and stuck.................On a horrible station!
Cannot change anything, at all. Sat nav still speaks loud and clear when appropriate.

Full disclosure: LH pollen filter cover has been giving me fits as i have tried several different types of closed cell
rubber to get a new seal on it. No luck yet. Thus, there has been water trickling down onto the LH side. I have not removed the kick panel yet.
On the Borrego, I am thinking back by the amp? Where does the "Telephone" signal come from? I am imagining a loose plug that is getting damp?

There's a connector behind the LH kick panel that carries audio wiring. It's an 8 way plug and I think on a non-DSP system it just carries audio but may well carry other things on the DSP system. Got to be work a look.

Dang!!
Started with Bolt, and pulled the LH Kick panel off. Plugs are clean and shiny, with Silicone grease intact, so no corrosion.
Pulled the head, and checked all of those plugs.
Plugged back in, and stone dead........
Switched to Borrego.
When I got in and turned the key, the radio came on....for about 5 seconds, then it said "Telephone" and refused to respond any more.
I pulled the sub, and DSP out and checked all connectors. All perfect.
Plug for phone is clean and dry as well.
Swapped the DSP since I could, and no change.
Then I took the head units and swapped them over....Now Bolt's stereo says "Telephone" and
The Borrego has a stone dead head unit. Problem is 'inna heads.
So, I have concluded that these are "Fair weather" head units, and that freezing fog is something they are protesting about. Tomorrow, I will take the heat gun to both and see if it is a wet or cold (or both) issue.
I am thinking cold solder joints which in the cold are contracting just that bit extra? Add a bit of salty fog....WCPGW?
So.....who is servicing the "High Line" head units these days (Both have good displays)
(Anyone got an DSP head unit or 2 they don't need?)