The Mercury Villager is based on the Nissan Quest, and if the two share the same computer and fuel system you can read out the diagnostic codes without any special tools because with Nissan Quests all you have to do is short a pair of pins inside an underhood connector and counting the blinks of the Check Engine light. The procedure is described in the factory manual, at least in the Nissan Quest manual, and also in the Mitchell manual that you can probably find at a library. The blinked diagnostic codes will not be OBD II numbers but Nissan's own, but each corresponds to an OBD II number. Chilton's and Haynes manuals provide far worse information about this, especially the hardcover Chilton's, which seem to provide nothing but a list only OBD II codes, not even any Nissan codes.