Ok, I tried tonight to move the power feed from the car harness to the M1ATX.
It worked of course but for starters had horrendous interference that was not present previously. Lots of scrolling horizontal lines for what its worth.
I tried to combat this by basically using both sources to power the screen. What I mean by this is that I now have it with the yellow (battery +v) lead running from the car's 12v battery harness, and the red (ignition +v) wire on the monitor powered by the PSU via a molex (yellow wire).
It has helped with the interferance considerably, by returning the main feed to the car harness instead of from the PSU, and in addition the monitor now behaves how I want, in that when I turn the PC on (I use a manual on/off switch for it from the mobo) the monitor turns on and folds out. Perfect. And when I turn off the PC the monitor closes. Perfect again.
Except there is a weird thing going on now. Basically there is now a situation somehow where there is power to the M1 ATX permanently! Ie, I don't even need the key in the ignition to switch it on, even though it is wired into the same supply it previously had, which gives correct battery +v and switched +v.
I have a suspicion that somehow the battery +v signal into the monitor (from the car 12v system) is finding its way via the monitor through the ignition +v of the monitor into the PSU and causing it to always have power. Does that sound stupid?
Be grateful for ideas anyway. It's sort of doing what I want but not quite....