Your best bet is to:
1. Consult your owner's manual: While it might not show a picture, it may mention the general location of relays or fuse boxes.
2. Check the fuse boxes: There are usually several fuse and relay boxes under the hood and sometimes under the dashboard. Carefully examine each one. The relay itself might be labeled, or you might find a fuse related to the horn circuit which could be near the relay.
3. Look for a diagram: You can often find wiring diagrams specific to your year and model of E250 online (sites like FordParts.com may have some information, but access may be restricted). These diagrams will show the relay's location. Keep in mind these diagrams are sometimes complex and take some interpretation.
4. Trace the horn wire: Start at the horn itself and follow the wiring harness back. This is the most time-consuming method but will lead directly to the relay. Be careful not to damage any wires.
Likely Locations (but not guaranteed):
Under-hood fuse boxes are the most common place for relays. It's often in a box near the battery or the power distribution center. Under-dash fuse boxes are less likely, but possible.
Remember to disconnect the battery's negative terminal before working with electrical components to avoid short circuits and shocks.
