Where do you add the oil in a ford 9 inch rear end?

There's no oil filler plug on a Ford 9-inch rear end in the traditional sense. The oil is added through the differential cover's drain hole. You remove the drain plug, add the oil through that opening, and then replace the drain plug. After filling, you then check the level with the fill hole, usually located on the top of the differential cover. Some aftermarket covers have a separate fill plug, but factory ones typically don't.