The topic of a backup light switch for the sm420 4 spd is brought up every two years or so. You just have to do a search like I just did. I have a sm465 in my '67 so it was never an issue for me.
In this first thread look for the post from "gmc248" at the bottom of the page and again on the 2nd. He has pictures of the original.
'66 switch On the next two threads, cmayna tells about a source for info and his own modification to put a switch directly into the top cover.
links His own mods (his first post, at top of page)
