So you are actually having a an issue, not just a code setting, since you say it starts in drive and reverse.
When you did the wiring originally, how did you set up your starter? Is it using the Magnum harness and relay or is it using the car's original starter relay?
That's a great question. I had a guy help me get it fired up and I honestly don't know. But I see where you are going, if it's using the original relay it would not be communicating with the NSS, correct?