Hey mate,
Reverse switch is normally on the driver side of the extension housing. Its a 21mm nut/sensor looking thing with 2x bullet terminals sticking out of it. Polarity doesn't matter when wiring it up. Depending what box you have the could be 1 or 2 other 21mm nut/sensors screwed into the top of the box, but they both only have 1x wire/terminal each so identifying the reverse switch (the one with 2x terminals) is easy.
Trust me mate, if your zuk orginally had a 1lt in it, then a 4k (and maybe more so the 5sp) is a serious upgrade!! ;)