Voltage reducer in G29 AC

[confused999]

I have a G29 with the AC motor, fleet version not PTV. Based on recommendations here I have obtained a DPI voltage reducer, 400A version which has the isolated outputs.

I understand how to hook up lights and whatever else to the output but want to be very sure I do the input right in order to not fry anything. I've looked at the wiring diagrams in the manuals for my cart as well as the AC PTV to see how the factory reducer is wired. It appears to me the positive input is wired from the output of the run/tow switch. Negative confuses me as it appears to come off a controller wire rather than direct to battery negative. Switched input is off the key switch, which on the PTV is the 3-position 4-terminal type i.e two separate circuits in one switch.

I would like to use the key switch as well but would like to still only have an on-off switch instead of the three position type, and I do not know if there's a key switch available like that with the 4-terminal setup.

Any help is appreciated. I know it's a different cart but I read about doing this wrong on the AC Ez-go and how that leads to destroyed controllers and other things so I want to get this right. I understand electric circuits and wiring fairly well but golf carts are new to me, especially this AC stuff. I also gather from reading here that very few own these.