I went through a similar aftermarket switch wiring thing and sorted it out in my 73 Valiant. That was a few years ago though. I do recall the switch had additional unused wire. Connect it as received and blow the inst' lamp fuse. Good luck with it.
Yeah the old switch had a small blue/white ground wire and the new one has a small black wire. The connector under the dash has a spot for it but there's no wire in there. I don't think a ground is necessary anyway.
