This thread might help somewhat:
I recently bought a Xenarc MDT-x7000 for my install. I am using an Opus 120W power supply and an Opus ITX vehicle case.
Previously, I used a switch from the battery to control the startup/shutdown of the Opus. This worked well and I could enter hibernation and resume within a matter of seconds.
However, now I have the new Xenarc with the front panel PC power button, I want to use that button for powering my system rather than having a dedicated switch like before. The Opus manual for setting jumpers, etc.. leaves a lot to be desired.
* The problem *
The system powers on OK using the Xenarc, but when I press the button to power the PC down, it instantly shuts down the PC before it's had a chance to enter hibernation. It just seems to cut all power.
My BIOS settings are set for S3, 4 sec delay, etc... (the system would enter hibernation before) and Windows is set to enter hibernation on pressing the power button.
I don't know if this is a problem with the Opus power supply (I've emailed them asking for their advice), with the head unit or with my wiring.