To find it, you'll need to consult your owner's manual. It will have a detailed fuse box diagram showing which fuse controls which circuits. Look for fuses labeled "Tail Lamps", "Rear Lamps", "Stop Lamps", or similar. If the license plate lights aren't working and a related fuse is blown, that's likely the culprit.
There are typically two fuse boxes in a 2004 Freestar: one under the hood and one inside the vehicle, usually located under the dashboard on the driver's side. The owner's manual will specify the location of each.