note: This is meant as an addendum to Bugbyte's Powering your Car PC when it's not in the car post.
Basically this is how I connect a spare desktop power supply to my opus for testing standby/hibernation/startup while not hooked up to the car.
Just make sure the power supply is up to the job, and that the power supply is turned on (green/PS_ON lead on ATX connector (pin 14) shorted to any black/ground).
Close the switch to simulate an "ignition on" in the car, and open the switch to simulate an "ignition off" in the car, which will start the delayed shutdown/hibernate/standby.
I don't know the pinout of the power input connector on other opus psu's, so you might have to rewire appropriately for your model.
also, for OPUS ITX MINI case owners, make sure to screw in the top cover before powering on the system. The power supply is grounded to the top cover, and unless the cover is screwed in an connected to the other piece (that the motherboard is grounded to), it can be a cause for problems like not POSTing and giving you a Bad Power Supply beep code.