1. Check the Basics:
* Trailer Lights: Are the bulbs in your trailer blown? Check all the lights (running, brake, turn signals). Replace any burned-out bulbs.
* Trailer Wiring: Inspect the wiring harness on your trailer for any breaks, corrosion, or loose connections. Pay close attention where the wiring enters the trailer.
* Trailer Connector: Examine the 7-pin (or 4-pin, depending on your setup) connector on both the truck and the trailer. Make sure it's clean and firmly plugged in. Corrosion is a common culprit. Try wiggling the connector while someone checks the lights – if they flicker, you've found a loose connection.
2. Check the Truck's Wiring:
* Fuse(s): Check your owner's manual for the fuse(s) related to the trailer lights. They're often labeled "trailer," "tow," or similar. Replace any blown fuses. Note that if a fuse keeps blowing, there's a short circuit somewhere in the system, which needs to be addressed before replacing the fuse again.
* Wiring Harness: Inspect the truck's wiring harness at the connector. Look for any damage, corrosion, or loose connections. This is often where the problem lies.
* Trailer Brake Controller (if equipped): If you have a brake controller, make sure it's properly connected and functioning. A faulty controller can affect trailer lights. Check its wiring and fuses as well.
* Ground: A poor ground connection on either the truck or the trailer can prevent lights from working properly. Check for clean, tight ground connections at both ends.
3. Test the Truck's Wiring:
* Multimeter: Use a multimeter to test for power and ground at the 7-pin (or 4-pin) connector on the truck. This requires some basic electrical knowledge, but it's the best way to pinpoint the problem. You should have power at the correct pins when the lights are activated. A wiring diagram specific to your truck's year and model is highly recommended.
* Test Light: A simple test light can also help trace power and ground.
4. More Advanced Troubleshooting:
* Wiring Diagram: Obtain a wiring diagram for your specific 2004 F350 model. This will show you which wire corresponds to which light function, allowing you to more accurately diagnose the issue. You can often find these online through repair manuals or forums dedicated to Ford trucks.
* Professional Help: If you're not comfortable working with electricity or you've checked everything and still can't find the problem, take your truck to a qualified mechanic or trailer repair shop.
Remember safety first! Disconnect the battery's negative terminal before working on any electrical system. If you're not confident working with electricity, seek professional help.
