Per Bill Hanlon's comments above, I did this a few years ago to my SM420 and it still working like a champ.
You can see the switch towards the front of the gearbox with a couple wires attached to it
And a pic of the backup lamps as well……..
Craig
My '50 Chevy 3100 5 window, '62-235cu, 3:55 rear My truck ....... Respect The Rust If I'm not working on my truck, '65 m00stang or VW camper, I'm fishing with the wife or smoking Salmon.