reverse light wiring question

Automatics up through 68? 69? had the reverse switch on the column/ shifter. When the 3 pin neutral switch came about, that is your neutral / reverse switch after that time

I would think fabricating a switch onto the linkage would be fairly easy, unless you have a later model transmission swapped in.

As ^^ suggested above, go to MyMopar and download the shop manual/ wiring diagram that closest matches your year/ model