the field wire is either the yellow or the black/red. mrshin says it's marked on the alternator. Other wise, start the car and put +12V onto each of the little wires coming from the alternator one at a time. When you get the right one you will hear the idle drop 1-200 rpm and you should be able to measure about 14V on the battery. If it still won't work you must have a dead alt.