There are switches that are operated by the (floor) shift lever, Push button type. 60's and 70's trucks have them. When the shifter is moved to REV it pushes on the switch and lights the light. Any momentary, push button, normally open, switch will work. Have to crate a paddle/lever to trip the switch mounted on the shifter or use linkage or shift lever under floor. Here is our buddy Jim Carter's article about a pretty rare switch in 55-56 on suburbans with a picture of the type switch I am referring to. I had the same switch set up in my 71 Blazer, under the boot.
http://www.oldchevytrucks.com/blog/?cat=613
