The best way is to use the plug provided. It should be hanging on the right-hand side behind the operator's platform. It is what the NH rear worklight plugs into. You will need to install a switch inline with your new light. I'll look up the details in a little while and post back with more info, but the short story is that NH provides a connector harness and all you have to do is tap into it.
EDIT: Yep, after posting, I went out and checked. On the harness going to my right tailight, they had a pigtail tied back with one orange and one black wire. The orange wire is hot anytime the ignition switch is ON. You can use the black wire for ground or wire your light's ground to the chassis. The connectors are round bullet type connectors.
