What do you use now as an amp turn on?
Does the opus 150 have amp turn-on? The M2-ATX does. With the m2 you just run a wire from this amp turn on to the amp turn on on the amp.
One way to do it is just to take the positive 12v from your car, and run it right to the amp turn on. Then put a switch in this wire. That way when you turn on the switch, the amps turn on. When you turn off the switch, the amps turn off. this is what I plan on doing so that I can have my stereo on when the car is off. You could also put a relay in that if you like.
I hope this helps.