Here is a quick set of diagrams for the A/B switch setup.
The switch that I got was from Amazon:
https://www.amazon.com/gp/product/B088PQW5DY/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Fits nicely in the rubber knockout pictured above the big plug (I have modified this box for air lines for an S30 spindle)
Switch from Amazon with jumper. Orange wire from Pin 33 in controller goes to either 3 or 4 on the jumper side (left) making it the common side. 4 on the right can go to MI1 (Run forward) and 3 can go to MI2 (Run reverse)
Original setup with Avid Plug and Play system
With simple A/B switch
With the switch from Amazon
Hope this all makes sense.