Hi
Tufty's comment is another stumbling block. You have to reverse the wiring with the controller
TURNED OFF.
So here we go...............
When in it's "normal" state, the wigwag will give forward motion for a particular movement of the pot.
In it's "reversed" state, the same movement of the pot will give reverse motion.
But the sequence has to be
Switch ignition on for normal drive.
Switch ignition off.......operate reverse switch.......switch ignition on for reverse drive.
The picture shows the pot and it's THREE wires.
Normally
wire A goes to Tag 1
wire B goes to Tag 2
wire C goes to Tag 3

To reverse the controls, a switch or relay is required so that it changes over so that
wire A goes to Tag 2
wire B goes to Tag 1
wire C goes to Tag 3 (and must not be altered)

You can see from this pic that wire A will go to lead 1 with the switch in one position, and to lead 2 in the other position. Similarly for lead B.
I hope that this makes sense so far.
How you actually achieve this is up to you.
You can use a switch, and simply turn the ignition off, then operate switch, then turn back on.
Alternately you can use an additional microswitch under the throttle, wired in series with the ignition switch so that releasing the throttle pedal turns off the controller, (but not the other functions such as lights).
I used a relay to do the reversing, and operated the relay from switches on a "gear lever".
These switches are arranged so that as the lever is pushed forward it puts the controller power on.
When the lever is put back in neutral, power is turned off.
As the lever is moved into reverse, it operates a switch, which operates the relay, before a second switch which then powers up the controller.
Take it one step at a time and you should get it sorted OK.
Best Wishes
Walter