Might there be a 3rd fuse box on our 2003 F150 4x4 7700 truck. The towing package is listed as an optional and we have read that some Fords for what ever reason have a 3rd fuse box not mentioned in the book that came with it

 

On the driver side you have the main distr. box, behind that there's the relays. Behind THAT there's a couple fuses in a small black taped box. The Relay box has to be removed to access these fuses. If the harness ain't working... check those fuses!!! So says the voice of frustrated experience :P

 

Thank you
This is our dilema:
We plugged the 7 wire plug from our 5th wheel into our 2003 4x4 F150 Ford and the lights all worked on the 5th wheel. We then wired the 4 wires from our small boat trailer into a 7 wire plug and plugged it into the 7 wire receptible on the Ford and the lights worked on the boat trailer. Replugged the 5th wheel in again and the lights don't work properly , both signal lights flash weakly when only one is turned on in the truck, the light on the brake controller comes on but the brake and tail lights don't work . we were at that point sooooo ticked off . Plugged the 5th wheel into our Chev 4x4 and they all still work on the 5th wheel . Didn't plug the boat trailer into the Chev cause we were afraid we might short something out on it . At least the Ford will pull the boat trailer and the lights will work and the Chev will pull the 5th wheel and we will have lights BUT we want to be able to interchange trailers and vehicles .
We checked the harness on the Ford everything is working except the power to charge the batteries in the 5th wheel. Managed to find it in the Maiin distr box and after a2hour drive to our closest center that carries these type of fuses we have replaced it but will wait till tommorrow to try the trailer on the Ford enough frustrations for today. Can't see how that will solve our problem but who knows. Appreciate your info in case we have to dig deeper for a cure

 

If the lights are weak, that sounds more like a ground issue on the trailer than with the tow vehicle. I'm not an electronics guy though. The best thing is to pull out a wiring diagram and start tracing circuits JUST in case there's a break. I'm assuming there's continuity throughout all the lines?

 

Well the lights on the trailer work perfectly on another vehicle and the boat trailer works perfectly on the Ford that the 5th wheel doesn't. So it certainly sounds like a ground issue . We too are thinking that it maybe in the trailer cnnector that for what ever reason the ground isn't now making connection on the Ford (but it was a week or so ago) and it is on our other vehicle . Seems Wiring has a mind of its own .Will let you know how things go. Thanks for your input