Tobal, check out the "Electrical" chapter of the Service manual on the site, I think it's in the left column of the home page. It has every single conduit by number, and lists each wire by color and start/end point. It will help you locate the proper wires to interrupt or jump into. Full wiring diagrams are also in there for each major system in the car, such as ignition, lighting, etc.
The DEI install manual should tell you which wire to grab for the install and that together with the diagrams should get you there.
I've never installed an alarm in the NSX, so I don't have any first-hand experience on it.
Don