* Under the hood, in the fuse box: This is the most likely location. The fuse box is usually located near the battery, often on the driver's side fender well. The relay itself might be labeled, or you might need to consult your owner's manual or a wiring diagram to identify it.
* Under the dash: Less common, but possible. Some F-series trucks placed relays under the dash, usually in a smaller fuse/relay panel.
To find it:
1. Consult your owner's manual: This is the best starting point. It should have a diagram of the fuse and relay box locations, and may even specifically identify the horn relay.
2. Check the fuse box under the hood: Open the fuse box cover and look for a relay that's either labeled "Horn" or has a symbol resembling a horn. Many fuse box covers have a diagram showing which relay goes where.
3. Look for a separate relay panel: Sometimes there's a separate panel for relays located near the main fuse box.
4. Use a wiring diagram: If you can't find it visually, a wiring diagram specific to your 1990 F-150's year and model will show the location and path of the horn circuit, leading you to the relay. These can be found online through forums dedicated to classic Ford trucks or automotive repair websites.
Remember to disconnect the negative battery terminal before working with electrical components to prevent short circuits.
