The main harness was the same, what was missing was a way to plug into it. What I don't recall is if we bought that short jumber that goes to the NSS, or an adapter harness that let the short jumper plug into the main. All I am really sure of is previous to that swap, it was unplug one and plug the other in with no missing parts and we had that little one I posted above left over. If we could find a pic of the reverse light connector on the side of an AX-15, that would go further to help.
Every AX15 I have ever seen is a 2-pin female connector (male pin) weatherpack. NV3550 appears to be the same.