Guys, those truck/heavy equipment truck switches are designed to go between the ground and the neg battery terminal. The reason is that under those conditions, the worst that happens when the switch fails and desintegrates (and they do... believe me!) the ground disconect install just does not... disconnect... the juice just flows from the terminal (now loose within the metal -grounded case-maybe not full amp cap, but contact just the same ) , but... if it is hooked up as a positive switch, and the same happens, the internals will bring 1000+ amps of 24V electrical power to ground... pronto... and fire then ensues... ask me for details... I had a master ground disconnect for my fire deuce and a positive disconnect to handle the electric primer and accessory pump and roll pump... one casual evening trip became a fireworks show when the internals of the second switch grounded out .... almost lost her... but quick thinking (not mine.... obviously) and quick activation of the first master disconnect... allowed me to continue on my trip with only a redder complexion and seeking the purchase of a second battery disconnect that now steps inbetween the ground of the primer/pump and roll pump and the chassis. Lesson learned!
Good luck