1995 Mercury Villager Villager wont start-starter & bat

I have a 1995 villager. It wont start now. All of the lights stay on, the door "chimes" when open, and the power locks, windows and radio work fine. It started out as: not being able to start Every once in a while, but then it would eventually start after I put the car in nuetral OR I do nothing and it would start on it's own. This problem occured for maybe 3 weeks off an on. Then, the problem stopped completely and it would turn on with no problem it was running perfect for the past 4 weeks or so. But two days ago it would not start again, and it wont start at all now! I had starter, alternator, and battery checked all are good and/or brand new. The spark plugs and wires, distributoer, etc, are all new. The thing I am thinking of is the ignition switch OR the nuetral safety switch / manuel lever position sensor, is one of these the likely cause? If so, where are they located? And how would I replace them? I do not have any more money to be wasting, as I have replaced a bunch of nerw parts already, and I was just laid off this morning! I need this van to transport my ill son and his required & bulky medical equipment. Please help me figure this out!