Welcome to JA.
Does the fuse blow with it hooked to the trailer or without it hooked to the trailer?
Fuse 14 is for the trailer side lamps so you either have a shorted wire on the trailer or in the vehicle. Check over your trailer to see if you see any chaffed wires shorting to the metal frame or what have you. Check this first since it would be easier than checking the car. If you have another trailer you could try hooking up that one to see if the fuse still blows. If not shorts found remove the left rear talight. Has 2 phillips screws and just pulls off. Take a flash light on look down in between the bumper and the body and you will see the trailer wiring coming up form underneath the car. See if you see it rubbing on the body chaffing the wires. If nothing there lower and remove the spare tire you will see the harness running along the top of the frame at the rear of the car check for more shorts on the harness all the way to the trailer plug. If you have a short blowing the fuse more than likely it will be in the area I told you to check. Land Rover has no diagrams for the trailer wiring so it is a pretty much figure it out yourself thing as far as they are concerned. If you dont find anything with the shorts I would replace the trailer wiring. There is no control module on this vehicle for the trailer towing. It is controlled by the central junction box.
If you are not getting power to the harness then yes your problem would be within the car harness. The only thing that you could do is trace the wires in the car you have to have a chaff or the whole harness is cut somewhere. I just looked one more time and they dont have a wiring diagram for the trailer wiring. Wish I could supply you with more info but that is all I can get since Land Rover doesnt ahve it.
That is where it is best to check in the inside. The only other thing I can tell you is to trace the wires back because LR does not supply and connector info or anything about the trailer wiring. You will just have to trace them back to the central junction box which is the fuse box/bcu. If you want me to opt out to see if someone else can help you I will since I dont have anymore info to give you and neither does LR.
It might be 2 seperate problems unless it is all inside of the junction box. If everything shorted together inside of it then it would cause this. There is no real easy way to get the connectors unlatched on the back of the CJB but drop down the glove box and remove the lower panel underneath the glove box it has 2 screws turn half turn to take and and disconnect the light from the panel. Removing this will give you room to look up from the bottom of the CJB. Is the floor wet on the passenger side or have you ever had a problem with a water leak if so along the entry guard on the passenger side there is a harness and when it gets wet the splices in there corrode and break. Pull up entry guard on right front just clips in just move it out of the way. Then you will see another black plastic cover with 2 white clips in it. the cover goes up underneath the carpet and a little underneath the seat. Pop those 2 clips up with a screwdriver to gain access to the harness. It takes a little effort to move this cover out of the way you can just bent it over to have room. The bulk harness will be in the channel that runs beside the entry guard cut the tape on the harness and check all the the splices they will be wrapped in blue tape and you might have to dig for them. Give them a tug to make sure they dont break and if they do repair them with butt connectors. Not sure if any of these splices are for the lights but I do know there is one in there for the keyless entry rf reciever and one for the power windows but there are a few more and if you have had a water leak they could be damaged so you could chek that also.