Wiper System Question

My 66 Dart owner’s manual says I have a variable speed wiper set up.
The variable speed wiper switch has been tested it as per manual and is working.
The wiper motor is a two wire signal speed. I’m guessing it was replaced at some time. Note: There are the four wires on the wiring harness for a variable speed wiper motor.
I found a variable speed wiper motor (four wire) Tested as per manual and it is working. I installed it and only one speed works.
I did a continuity check of the wires from the switch to the motor and all are ok.
I’m thinking the ground on Motor and the live wire from the ignition switch must be working because the wiper motor is rotating just not at variable speeds.
I’m stumped!
Something I find really odd on the wiring diagram in the manual is that one wire goes from the wiper switch to the backup light switch. I can’t think what these to electrical circuits have in common.
Any suggestion as to why the variable speed is not working?