We have a 2007 expedition el. I got the tow package because I tow with it. I have three trailers that I tow with the truck. The small trailer and boat trailer use a 4 pin plug. My tandem axle trailer I use to haul my IH scout 4x4 uses a 7 pin trailer. I noticed my lights were not working on the big trailer and I was not getting power to the turn signals and brake lights on the 7 pin plug on the truck. I get power for running lights. In any event I look at the fuses and find a blown 25 amp fuse that controls the turn/brake lights on the 7 pin plug. I am going to replace the fuse and see what happens but any ideas on what could cause the fuse to blow would help a lot. Do you all think I have a problem with the trailer wiring? If so is it a ground problem. I need some help. BTW I do not have a brake controller yet hooked up.
I would start with the trailer wiring. You can check your ground on the ball as-well. You may need to grease it depending on your trailer. I would also take a look at the plug on the truck and make sure that all of the wires are secure into the back. If you are looking for a good controller I just bought a Tekonsha P3 and it works like a champ.
Assuming you have the heavy duty tow package, the trailer plug wiring has its own isolated relay isolated circuit and fuse protection. This is a very nice feature as you just found out that you only blew a fuse on the the trailer lights and didn't loose all your turn signals on your vehicle. Look at the wiring diagram that is with your brake wring pig-tail (check by your tire tools & jack) for the relay & fuse info. You should not rely on the trailer ball for a ground. Use a multimeter to check for shorts to ground on the trailer wiring.