Variable speed wiring

I had bought a beautiful chrome wiper motor from Jules for my 71 Challenger and had to find a variable switch to run it. I found one but the wiring for my car is for a two speed. How do I wire the extra prong in the back of the switch to make this run right?

Wiring is the same to all units, 2 and 3 speeds on B and E bodies. Difference is just on 72 and laters plugs due diff size and prongs distribution. 72 and later switches are smaller. But wires itself is the same, just plugs and prongs setup changes but is a year stuff not a speed stuff

EXTRA PRONG could it be ( not sure ) for ground reinforcement. doesn't plug to anywhere. Switch takes ground from chassis ( required ) 2 speeds doesn't need ground.

I just made the 3 speeds conversion on my 74 Charger from 2 speeds. I got 3 speeds 73 switch and 71 Variable speed switch. Decided to go with Variable since I got the right matching plugs for 71 setup.

simply use existing wiring and done

Thanks for the info.