Its gonna be the yellow wire, which is just a constant + going to your alt so when you cut it dont let it touch anything.. the diode should be 'forward', so since the alt is what is reading the voltage you want it facing the alt. A better idea looking at the diagram again would be to wrap two diodes in opposite directions and do it that way. The opposite facing diode would protect the alt if any current was to try to flow back into the alt.