At first you say that you are using the same battery that you were using in the house. If you use this same setup in the car, then this makes no sense, it should work. Then u mention a Shack battery???
Did u use the same 12g wire in the house? Check the wire, or use a different one.(I'd go bigger anyway)
Hook up a volt meter at the Opus, I bet the voltage will drop too low.
I'm not familiar with a Shack battery that would provide enough juice, <confused>
Bottom line is, it worked in the house, so the Opus is good. It doesn't work in the car? I'd say you have a problem getting enough volts to the Opus. Check power or ground.
Let us know what fixed it, hope I helped (some)