The fuse for trailer lights on a 2003 Ford Excursion isn't a single fuse, but rather depends on the specific lighting circuit (running lights, brake lights, turn signals). You'll need to consult your owner's manual. It will have a diagram showing the fuse box layout and clearly identify which fuses control the various trailer lighting functions.
Without the owner's manual, I cannot give you a definitive fuse number. The fuse box may also have labels, but relying on these labels alone might be insufficient. The owner's manual is the most reliable source of information.
