Put SM420 or Muncie 420 in your search engine and one of the hits will be for a Jeep parts place they have a page the details how and where to drill & tap a hole in the tranny cover so you can then install a standard BU light switch , when you
shift into reverse the rail comes forward and depresses the ball in the switch and closes the circut .
I'm sorry I lost the link and don't have the time allowed to re-find it .
