The kill switch wires on mine are one black and one green.The green one grounds to the engine block somewhere(I had mine on the gas tank mounting bolt).
In my case,the engine had a low oil shut-off(a little box with 2 wires on it),which I removed.This left 2 black wires from the engine(ignore the yellow wire.It's the low oil sensor).One went to the engine mounted switch,the other was unused.I hooked the black wire from the kill switch to this unused black wire and it worked perfectly.