Is it new or used?
It would be best if you could provide a pic of the kart
...& the switch (front & back side)
I would first "test" the switch, by just by-passing it (to see if its malfunctioning)
* In most of these little electric vehicles the F-n-R switch is just a (3) wire On/On switch
...which means when in the 1st position (like forward) it connects (2) wires (like maybe the top & middle wires)
...& when switched in the 2nd position (like reverse) it connects the other (2) wires (like the middle & bottom wires)
TBH
I'd like to see what you have, or are working with, before advising further
